How do I tell my SO that I (might) hate his family?

I have been in a relationship with this sweet guy for over a year now. We are in our early twenties so it’s not like we would be considering getting married yet or so… But we had talked about it, and we agreed on a wish to get married one day with the right person (we were still getting to know each other back then). Anyway, the problem is that we have a very different family backgrounds. The relations in my family are pretty much cold, I know only the closest relatives, one of my brothers is a NC to my parents and I mostly grew up as if I was an only child (my brothers are way older than me), so to me, my only really close family are my parents, I am quite independent and some things which really bug me about them, make me prefer to stay away from home, I love them, but it gets frustrating staying at home for more than a few days for me. He is a person who is bugged to not see his family for longer than a week, often stays home or leaves early for a weekend to be there, lives in the 3 generation house there, and has a wide family who meets for gatherings like birthday parties and other. And once a year, during summer, his family makes a big party where the wide family meets up, with music, fun theatre and lots of food. It has been an ongoing tradition for over 10 years and it seems to be kind of a big deal for most of them. We met soon after this party last year, so I avoided it till this year… And let me just tell you, it went worse than I and probably even him expected. As an outsider, I find it kind of off that they make a big thing out of it, but still do rehearsals like a week at most before the actual party. I wasn’t feeling confident about playing my guitar with them, so I agreed I would help with the fun scenes which accompany the songs… I was told what I would be doing, so I learnt/got prepared for that, but I wasn’t at any rehearsal since I was gone, working at that time. The day of the party turned into a mess, he himself claimed it was more chaotic than usual. There was a family meeting at the cemetery, where I was introduced to those from the further side of his family. Then they visited the family graves and sang a song for a man this party was invented for as a memory (yes, I didn’t know the lyrics, as he forgot to mention this until like during breakfast on that day). Then, the family was gathered to go to the castle nearby and like… Were invited to a wedding which took place there. I was confused, everyone was confused. It was planned by the groom and his siblings, to snatch the family, which would be meeting anyway for the party, to attend the wedding with his new bride. Among the family which gathered was his ex-wife, who divorced him because he cheated on her with the woman he was marrying that day. I just left with my man’s little cousin, we spent the whole ceremony outside, watching frogs in the fountain, as did some of the relatives and the ex-wife. My boyfriend felt embarrassed that I left away from the crowd like that and dragged me back. After that, everyone left for lunch to get the last things before meeting at the party spot, that was a moment when I learnt why it was even odder. I felt like if something like this happened in our family, it would divide the family and make them go NC. Then I got comments from the mother of my boyfriend, that I didn’t overcook the stuff for the party like she wanted them to be when they left to get something with his sister. And after we got to the meeting spot, I understood I messed up, cause I was in a strange place, surrounded by people I barely knew (yes, I am an introvert), my boyfriend was busy with preparing the programme and the fun theatre, that I basically didn’t get to talk to him much, I didn’t even know where the bathroom was, as he was too busy to give me a tour, or to tell me where should I put my stuff. When the first scene started, I was supposed to take a huge role in that, but when we were lining up, I ended up in the back and wasn’t sure when I am getting on the scene. So then I got a quite big backlash for that, from the far family, then from my boyfriend and his parents and their siblings. I felt so awkward and so bad about it, cause it was a one-time chance to do it right, and I ruined it. After that, I got to explore the place, so I could hide somewhere to cry to relieve what I felt. I texted my best friend, who understood that I felt uncomfortable in there, as in her words – family gatherings suck, even much more if it’s not your family. A few people I knew from earlier tried to talk to me, but I was way too upset. My boyfriend found me when I was mostly holding my tears back again, trying to drag me among the rest, to see another show. I passed. And then I joined him and his family for a second scene I was supposed to be part of, again, without any idea, when do I do what I was supposed to do (this time just dance). So I just stood in the back, keeping my mouth shut, not even trying to sing. Next comes, more backlash from his parents and their siblings for being too stiff, to take part in any way. My boyfriend was the one who was trying to shush them for it. I acted it out as if it was all fun. But as soon as I left the room, I just felt so horrible, I barely spoke to anyone after, occasionally leaving off to cry and shake off the cold weather. I just wanted to be gone from there as soon as possible. After the programme was over, and I had a few shots of alcohol (I didn’t want any at first, but I felt so awful, so I felt like alcohol could help), I was asked if I liked it (the party), I didn’t even try to lie, my boyfriend was dumbfounded by that. Tried to take me for a walk, but I got shaking with cold and so annoyed, so he took me to their home as the only sober person around. I just got there, took shower to help with the cold, and lay on the bed, hiding from light and listening to music really loud, he stayed with me and tried to talk to me, but I felt too distant. So I just didn’t say a word. He tried to make me feel better, but it wasn’t really helping. We talked about it the next day in the morning, he was shocked that I took the party so unwell, as his ex took it well (I HATE whenever he compares me to her), also saying that one year they had some Erasmus students around, who didn’t even know the language and enjoyed it. Ultimately asked what I think of his family and if I want to break up. I didn’t think much of anything, just that I felt like I should have thought it through more and not contributed in any way, as after all that happened, I felt like I ruined all I could. Possibly as well leaving the worst possible impression on his family… I didn’t want to break up. ’cause still… Despite not being there completely for me during the event to help me, he at least TRIED to make me feel better when he saw the mess I turned into.

From time to time, it is brought up again like what I could do for the next year, as I am definitely not helping, but I don’t want to be just sitting there in the corner in silence again. My current plan is to bring my own wine and drink the whole bottle myself, so I have an excuse to avoid talking and something to help me numb the anxiety. But recently he saw his family again, and they said I acted so very strange at the party, others asking if we are still together (I did push him away a few times at the party when I felt horrible)…

Well, like this, I have a confirmation, that my reputation among his family is ruined for good, so I am not trying to make a good impression on them ever again, I might never want to contribute to the party in any way in the future and I have been replaying moments in my head from that day last weeks and I came to the conclusion, that I might actually hate his family. Not him, I find him the loveliest… But the comments got on me so heavily, I feel like they might be reminding me of that during next year, or the following ones, since the party is an annual thing… thinking of it makes me want to scream at them, stuff like “oh, I haven’t been to any rehearsal, so I better not be ruining it for you all”, “oh, I have no sense for cooking/baking, so I am not bringing any food” or “no, thanks, I don’t think I could ever play in front of you all” in those memories… I know that I will have to go through this party again if I am staying with him, cause it’s important to him… But I am not sure if I am actually able to handle that 1+1 package of him+his family for my whole life if it will be always like this… 

How would you tell your love that their family might just be too much for you, despite you love them unconditionally?