I really fucked up this time

I have been in an off and on situationship type relationship with this guy for a year now (I’ll call him mark just for story reasons) We never fully dated but still we’re “together” in a way, the whole thing was complicated and I don’t really understand much either. We have a slight age gap which is partially why we weren’t ever together because of what other people might say. Regardless of all of that Mark was my best friend first and foremost. He wanted me to be able to be responsible and not so impulsive and be able to show self control before he would actually date me, because in his eyes “we’re just not compatible”. I loved him tho so I often begged and kept him there with me despite us not really going anywhere in the relationship despite hanging out and having sex. One thing I promised was to tell him if I ever had sex with anyone else, to keep each other safe. Sometimes I felt unwanted by him constantly telling me to find someone better suited for me, that were not good for each other, that my style is bad, etc. recently it had been okay and he had even come down to see me instead for a change (were slightly long distance). The trip was fine and I was happy, here’s when I made the mistake however. A few days after he left from visiting, I met a guy and he had asked me out on a date, I’d said yes because I’d known that Mark was still not planning on being with me really. And since we weren’t in a committed relationship I didn’t see the problem with going on a date. But he’s my best friend, I tell him everything, however I did lie to him about what I was doing I know I should’ve just been truthful, I just thought I’d spare his feelings because I love him and don’t want to hurt him. On the date I ended up having sex with the guy ( he kept talking about it I felt a bit pressured to go along with it, I still consented though and I take full responsibility for that) Doing it with someone else made me realize just how much I loved and cared for Mark and no one could ever replace him in anyway. I felt really guilty and so I told him the next day what had happened. He didn’t say much, asked a few questions like “when did this happen?” “Did you use any protection?” “What happened exactly” I could tell he was hurt. I knew going into it It would hurt him, I don’t know why I gave into the impulsivity. We’re not committed, but I feel like I was at the same time.he won’t really talk to me now, I know I should just give him time but it’s so hard because he was also my best friend,, and I ruined that too.
Extra context:
We were trying to move on from each other before back in January, but I’d decided I didn’t want to becuase I loved him, and so I went all out on Valentine’s Day for him and he said he didn’t have the heart to move on after that but now he thinks he really should. I know it’s my fault and I don’t expect him to forgive me. I hate myself for every decision I made that day, not only did I hurt the one I love but I lost him too. I hope to make up our friendship to at least talk one last time if he never wants to see me again, I do understand. I just miss him, more than a partner but as my best friend