Overreacting jealousy?

My boyfriend started having feelings for me in June of last year. It happened when I asked him to help me with something, and we got closer, becoming best friends (before we had been just friends). I really cared for him, he helped me at a difficult time, and we talked all summer despite being in two different countries. I, however, had no idea that he had started liking me as more than a friend. After reuniting in September in school, he asked me out and told me he liked me. I had to reject him, because of several reasons. I had been single for 2 years after a bad relationship, and I was caught by surprise because it never crossed my mind that he liked me, plus I was scared that if I said yes, we could ruin our friendship, since he was my best friend. He was extremely sad and even cried, and gave me the birthday present he had gotten from his country to keep until it was my birthday because he couldn't bear to have it with him. I thought he wouldn't want to talk to me again although I told him our friendship was so important to me that I never wanted to lose him. After not talking for some days, he told me that our friendship was also more important and we decided to stay best friends. For the months that followed he acted like the chillest person I knew around me, and I really appreciated it. He never made anything awkward and I felt super comfortable, despite what he had told me about his feelings. We actually never talked about that again. My feelings for him also grew but I didn't say anything because my previous relationship had gone so bad that I was used to the single life, and really content with having him as my best friend. Then in March we got even closer, started spending our evenings at each other's rooms and he kissed me, and was over the moon over the fact that now I finally returned his feelings. It has been an amazing relationship, he is my best friend and boyfriend at the same time and it is beautiful. There is only one problem, and that is my insecurity and paranoia, based on the fact that I was cheated on before. Especially because of one girl. She is a friend of both of us, whom he drunkely kissed last November while we were still friends and he was trying to get over me. They kissed twice, and after the second kiss he told her he couldn't because he liked me and she was fine with it, actually even asked him about me. He told me all this himself and at the beginning I was totally fine with it. After each month however I have gotten more jealous. She is an awesome girl, we are both friends with her but idk I just can't help it lately. They study the same thing, and do text each other to ask questions for their assignments, I have seen their texts and there is nothing to worry about but I also just feel jealous every time they come back together from class. I fully know they are friends because they were before the kiss and still are but I don't know how to stop the jealousy. Now, in the summer for two months I had been wondering if they had talked or not, and he told me they only talked once because she messaged him asking how his summer was going. It seemed like my worst fear had come true and I freaked out. He knows I am insecure and paranoid and understood how bothered I got and said he would not talk to her again, but I also know it's not right especially because she is a friend of both of us and I really like her. I just can't help the jealousy. I fully know he loves me because he has done so many things for me, and pursued me for a year. But I just keep picturing them kissing when me and him weren't even together. Fully knowing that he was trying to get over my rejection at the time, and after the second time he couldn't do it because he liked me. So idk what to do and would appreciate any comments/guidance/help.