I'm 18, I got my first boyfriend in January and we were together until April. I had a serious problem with retroactive jealousy about his ex, even though he told me he didn't want her anymore and I believed him. Even though I believed it I just kept feeling obsessive about her, imagining them together and wondering every time we did something if he'd done it with her and better.
We broke up for something unrelated and I thought the jealousy would go away, but it's still there and sometimes worse - tonight I looked at her instagram and of two girls he once had a thing with but didn't date. I even looked up the ex of a different boy I talked to for about 2 weeks before getting ghosted. I can't place what feeling it gives me, I just feel sick and sad and I don't know what's causing this or if it's normal - I feel insane and like I wish I'd just never started dating.