There's quite a few, especially from college, but two in particular stands out.
The first took place during a fall semester in college, what would be the worst semester of my life. I was in very few classes and wasn't doing well in them, plus I had a massive crush on a girl, so I was devoting a lot of my energy to that with having very few distractions to keep my mind off of her. The crush built up and built up and made me overly anxious and depressed, constantly trying to figure out if she liked me or not, if her extra attention towards me was because she thought i was a good friend or something more. I had no hobbies or no other interests in life except focusing on said crush. My crush on her was fairly obvious to everyone around us in our social circle. I thought she had to know that I had liked her, it seemed crazy that she wouldn't.
Anyways a weekend comes around where me and said crush are spending a lot of time together, and she drops the ball that she has been dating/hooking up with another guy for weeks/months now. I was emotionally devastated, and she honestly was taken aback by how bad I reacted. For some reason, she couldn't figure out that the reason why I was so upset was because I had liked her. We're still in a position to hang out a lot with each other one on one that weekend but it becomes more awkward and tense as it goes on. She is becoming upset by this, and I tell her "I can tell you the reason some other time why I'm so upset." She asks why can't I tell her now, and finally when it's just me and her alone again I spill the beans, everything that I've been feeling about her and the crush and the whole 9 yards. She's honestly suprised by all of this, saying she had no idea, and rejects me. From there, we still had two days where we had to spend a lot of one-on-one time with each other, and it just became more awkward and tense throughout until a huge argument at the end. That was the first bad memory.
I left that weekend feeling very depressed and melancholic, and for the rest of the semester became a drag to all of my friends. My grades suffered and they became very distant from me, not inviting me to things anymore, and sometimes becoming openly hostile towards me when I tried to engage them (not about the girl, but just in general.) I constantly yearned for their acceptance and friendship, pathetically so, but was often left with no results. I left that semester going into winter break feeling very alone and fragile, and wanting to kind of get out of my funk and sort of "transform" myself before heading back to school that next semester. I was really making a lot of progress, reading inspirational books, working out, shifting my eyes onto other girls, all that good stuff., but still a big part of me was very hurt from the experience with the girl, and the resulting social exclusion that came as part of it.
The day before I was supposed to go back to school, I had gone to hang out with a couple of guy friends, one of which was a really good friend from home who also went to said school. This guy had very much encouraged me to be the "stupid, goofball" type when introducing me to his friends at school, and I quickly adapted into that personality as a means of getting people to like me, being remembered for something by doing stupid things instead of being remembered for nothing, and said friend was the primary encourager of such behavior. But he was also the main person I would confide in about my struggles with such girl, about my sadness over the other group not liking me anymore, and he was generally pretty helpful. On this final night we were hanging out before going back to school, we ended up goofing around, racing each other in our cars or something like that. I get out of my car and hock a couple of loogies on said friend's car, and he becomes livid. Me and my other friend get back into our car and drive away, high fiving each other because it was so hilarious, things like that.
I look at my phone and see I have three missed calls from the friend at school. he calls again and asks "what the fuck you were doing, etc. etc." But then he says passionately "this is why! this is why burner_9898!"
"This is why what?" I reply, already knowing what he's about to say.
"This is why everyone hates you!" he screams.
From there, he yells at me that I need to clean up his car, and I wait in my car, curled up in a ball for 45 minutes waiting for him to arrive in a dark pit of despair, reflecting upon everything that had happened to me over that semester. He finally shows up and I clean off his car as he watches with a big grin on his face, and then we went back to our respective houses to sleep before heading out on our 16 hour drive back to school together the next day. On our drive I'm visibly upset, he asks me what is wrong, I tell him I'm upset that he said that, and he denies ever saying anything like that.
That was the second bad memory.
That following year would be a repeat of similar situations, trying to gain acceptance into that social circle but being constantly rejected or left out, and continuing to do stupid shit as a means of gaining their laughter and hopefully their acceptance as a result, but never their respect. The experience with that one girl would make me a social pariah among the females, and my constant yearning for acceptance would make me a pushover to the guys. When I finally left, it was one of the happiest days of my life.
There were a lot of other bad memories during those years. I never saw the girl again after college, and I would later go on to sever ties with that one friend and pretty much everyone in that social circle after coming to the realization that he was never really good friends to begin with. My life has improved a lot since then, but still, years later, I still have confidence, trust and depression issues as a result of those experiences. I've done some therapy and have been able to cope with those memories when they pop back into my head, but they still hurt very much to this day.