My boyfriend and I are very open with each other. We have talked about our past crushes and who we thought were cute. One of them is one of my best friends. I would have no problem with this, but he had masturbated to her when he liked me. He said he did it to take his mind off me. He was obsessed with me then. He said that she was a distraction from me.
During that time, we both had a huge crush on each other. I wouldn’t have minded if it was any other girl, but this is one of my best friends. Recently, he admitted that he didn’t know if he masturbated to her just because of her appearance. When we first discussed this, he said it was because she reminded him of me. Now, I don’t know what to think anymore! It hurt me a lot. I thought I was the only one he thought about, had a huge crush on at the time. Whenever they’re in the same room together or talking, I get jealous. I don’t know if he still thinks of her that way, because recently he said that it’s human’s nature to be attracted to other people too. I’m also hurt because he once said that if their age gap (3 years) wasn’t so big, he might have pursued her. I know he loves me very much and he has tried to assure me that it was nothing, but I can’t stop obsessing about it. And I can’t quite forgive him for the fact that he masturbated to her while he liked me. Please give me some advice!
LemonVibe 
Ok so when you were not dating he masterbated to the thought of another girl, admittedly a friend of yours, but still when you were not dating yet.
That’s normal. I know it becomes more personal, because it’s someone you know and you are entitled to feel the way you feel, but you need to let this go.
You are young, and I can tell because you said three years was too big a age gap, so I think it’s time you started educating yourself on what’s normal for sex and masterbation. Once you do you will understand this isn’t a big thing at all and feel better. Maybe start at https://sexetc.org and look up books in your school library. The librarian isn’t going to judge you for educating yourself other than to be proud of how smart it is to do so for a young woman, so don’t be bashful about checking stuff out.
Remember, he chose you for a reason.