Advice please ASAP!

I'm 16 my boyfriend of a year and a half is 17. Recently there's this girl named L that he's been talking to.

Before I start- this will sound silly and petty but I'm 16 and been cheated on multiple times in the past and have anxious tendencies.

Anyways, L and him are really close. My boyfriend, D and I talk hourly and all the time on Snapchat. Every few months his #1 best friend will change to L for a long time because they talk more than we do. He told her first about some of his personal things, they talk about deep stuff and are really close. Which is fine- I know they are just close friends and trust my boyfriend. Guys can have female friends. But then when I asked him how much they talked and what about, he lied to my face about it until I went further into it. He said he was telling me what I wanted to hear.

Backstory- I just confronted my abusive ex of a year about abuse/SA and have been really caught up about it, and told D not to talk about it without my position. I found out he told L everything without letting me know. And when I asked him, he said he didn't at first until I asked him how she found out- so he lied to me.

Then though, he gave her a really expensive designer sweater for a little while. It's been a few months now and hasn't asked for it back or anything. That was also weird to me.

Lastly- I just found out they used to like each other last year/two years ago. He said he liked her for a few days after she gifted him for Valentine's. I did not know this. I also found out she liked him for a full school year, but didn't work out because they had feelings at different times..

I was feeling really insecure about this and wanted to ask you guys your thoughts. I understand it's not that deep and I do trust my boyfriend- this is just making me uncomfortable. I voiced my opinions to him and he didn't really say much but told me he loved me and that he was sorry for making me uncomfortable after saying he didn't understand at first.

what do I do?