My (24m) girlfriend (22f) drunkenly kissed my best friend (23f)

Alright, I’ve never done this before, so here goes nothing.
TL;DR: Last night, my girlfriend of soon to be four years, let’s call her Nicole, admitted to me alongside one of my best friends, let’s call him Sebastian, that they had kissed during a concert at a bar, while I was off buying both of them a round, two weeks ago. Now, too be fair, they were both smashed out of their respective gourd’s, during this. Sebastian is currently coming out of a four year long relationship as well, and I think he’s been pretty scatterbrained, as he made the first move on Nicole, however she admitted to reciprocating.

For full context, Nicole and I haven’t always had the best relationship. We’re both very passionate, stubborn people, and we used to argue quite a bit, and some arguments were way worst than others. I say used to, because I’ve recently been diagnosed with PTSD, and Nicole has actually been very helpful and supportive of this. She’s actually been amazing, and over the past nine months or so, we get into less squabbles, and when we do, we’re able to manage them effectively, I really feel like we’ve both grown together.

With sebastian, just after graduating high school, we had stopped being friends for years because my girlfriend at the time had left me for him, only for her to leave him for somebody else just a month later. It was a really big issue at the time, and sebastian and I have rekindled our friendship since early 2020.

Now with that out of the way, I want to explain what’s been happening. Ever since the concert, Nicole has been telling me that she feels we’ve been growing apart, that we’ve been settling for each other, but I haven’t felt like this at all. Last week, for five days, Nicole had a vacation to see her family in Texas, she had brought these issues up before she had left, and she had brought them up again the day after she got back. However, after she got back, she’s felt that’s she’s been on a really downward spiral mentally, and all she wants is for me to be happy, and thus could be a result of her having to up her medication for her own depression. I’ve been constantly telling her that I’m absolutely happy with her, and I wouldn’t trade anything for her. Again, I’ve felt that over the past year, our relationship has greatly improved, and that I’ve been so incredibly thankful for all she’s been helping me with, and it’s made me find a whole new love for her.

Things changed yesterday, however.

I had spent the first half of the day going to an unexpected funeral for an old friend that had passed away just a week prior. I say unexpected because there reportedly wasn’t going to be a funeral until the night prior, my second half of the day I had spent at work, and it had been a surprisingly busy/difficult Tuesday for my bar. In between the funeral and my work, I had gotten lunch with Nicole, and spotted on her phone that she had been texting her best friend something about breaking up with me. When I asked her, she immediately hid her phone and attempted to pretend it was nothing, but then admitted to saying we may need a break because she feels were growing apart and that she hasn’t being feeling well mentally, and that she wants me to be happy. I repeated myself and told her that I’ve been happy with her, to which she was glad to hear and decided that she didn’t want a break. I still felt uneasy about it and that she was hiding something from me, and that she wanted to break up.

After work, Nicole had picked me up, which was normal. What wasn’t normal, was that both of my best friends were there, sitting on my couch. I immediately began freaking out, as it felt like I was in an intervention, and didn’t understand what was happening. Eventually they calmed me down, and explained that her and sebatian had kissed, and Nicole had told me the night prior to the concert, she had a dream of Sebastian sleeping with her sister, and felt jealous because of it. They had both admitted that they have small feelings for each other, and that they’re not sure if it’s fleeting or not. I’ve been cheated on before, and honestly, Nicole was the LAST person in the world I expected to cheat. Everything about her screamed loyalty, and she’s always felt that cheating was a cardinal sin, and was a full deal breaker. I don’t know what to do, were all good friends, we all play D&D together every week, this is my circle of friends, my weekly routine, I look forward to my weekends to have my closest friends over, and now I feel I can’t trust one of my best friends alone with my girlfriend. I know how feelings go, I can’t just tell them to turn it off, in fact, trying to fight feelings just makes them stronger.

If it were just a kiss, it would be hard, but I could forgive it, I understand that people make mistakes, but I’m not comfortable with the idea of them possibly having feelings. I don’t want to lose my friends nor my relationship, but I don’t think I’m strong enough to let the idea of the possibility them having a crush on each other.

Edit: my best friend is a male, 23m, my bad, and I don’t believe I can change the title, at least on mobile.