I need help.

Hello I'm writing here because I'm tired of myself. Does anyone know how to stop feeling jealous and insecure of boyfriend's past? I developed this habit long time ago which is why we always argue because I mention his ex quite often. He could say something related to music and I would connect it to his past if he knows it because of her etc. It's really dumb and upsetting. I can't stop these feelings because when things are good, it always comes back out of nowhere. It doesn't help knowing she was similar to me in terms of looks and interests. It got to the point of stalking her a lot and also going through his phone and computer and scrolling back to see what he was watching/searching/listening to while he was with her and connecting it, then arguing. I also want to say because of me always getting mad, he changed some stories related to her and then I would find out about and I would find it hard to trust him again. We really love each other and he reassures me like crazy, says that he never loved anyone and I'm the most important to him. I know this is very unhealthy. I don't want to lose him and I know that it's exhausting and draining to deal with this. I also want to say that it might not seem like a big of a deal to others but it is very harsh on my mental health

I'm sorry you're going through this.
Members here have recommended two relationship phone apps. One is called "Lasting" (recommended based on experience), the other is called "Paired" (recommended based on reading a review). I can't guarantee that the phone app would help, but it might be worth a try.

In-person couples counseling might be another option, but it's a bigger investment in time and money. Individual therapy might be of some help too, though also similarly a big investment (unless it's covered by insurance).

CBT might be good for you. There's evidence that you can get help from learning about CBT from a book. This book is one of the most highly recommended CBT books for depression:

Mind Over Mood, Second Edition: Change How You Feel by Changing the Way You Think
Dennis Greenberger, Christine A. Padesky

Maybe learning CBT techniques will help you to think differently about your relationship.

The treatment methods in the links (to pages on SF) below might help with your mental health overall, and therefor also help with your relationship:

Chinese Herbal Medicine and Acupuncture, World's Second Largest Medical System

Self-Treatment and Miscellaneous

Thank you a lot, I will try those.