How to be a better lover?

My ex and I met when we started university, and after a good friendship, we got into a relationship. However, he had a past where he used to flirt with his female best friend in a very close and romantic way. Before we got into the relationship, he didn’t mention his past, whereas I had clarified everything about mine. A few months later, he revealed his past chats with his female best friend, and after seeing those chats, my expectations and heart both broke. I started feeling insecure because he didn’t treat me the way he treated his female best friend. He did some really special things with her but not with me. After learning about all this, I started comparing myself to her and began feeling very low and insecure. I couldn’t love myself, and every time my ex did something for me, I felt it wasn’t exclusive to me, whereas many things about me were exclusive to him.

I started behaving rudely with him, and over time, my behavior toward him got worse. He was a nice guy, and he tried his best to make me feel special, but because of my mental state, I couldn’t accept those gestures.

Recently, my ex broke up with me, and it's been 21 days now. Our relationship lasted almost a year. He says he was mentally frustrated after dealing with my tantrums and mood swings. After the breakup, I assured him and requested another chance, promising to treat him better and seek therapy. But he’s afraid I might repeat those behaviors if he comes back to me. This fear is keeping him from reconciling. He repeatedly told me to get therapy and help, and I’m willing to do so. He also said that if he feels it's better for him to come back into my life, only then will he consider getting back together. He hasn’t blocked me and still replies to my texts.

I am suffering from severe attachment issues. My mental state ruined what could have been a perfect relationship. Can someone please suggest how to fix things between us and improve our mental health?