Why am I feeling this way about my ex?

I broke up with my ex boyfriend who was my first love. We rushed the relationship and we only talked for a month before becoming official and then a couple weeks later we told eachother we loved eachother. We fell inlove so fast but that was the issue for me because when he showed me his true personality and I lost feelings quickly.

I broke it off with him and we both agreed to stay friends but he was too in love with me still and I was so confused and annoyed that we got in a fight and blocked one another. I was so happy after this actually. But a couple weeks later he was constantly on my mind. Lately he’s been popping up in my mind and i ask myself “what if we were still together?” “Do I miss him?”. But if I were to talk to him again ik I wouldn’t ever feel the exact same as he did for me so I knew I made the right decision.

As I still think about him on the Daily because he was a good part of my recent life. Also, me and my friends have a inside joke thinking he’s “gay” cuz of a certain event. So I’m obviously over him because Every-time i talk about our relationship I just regret it.

But I then get sent a picture from a friend of him kissing his new girlfriend at prom. I was hit with so many feelings I didn’t know what to think. At first I felt like I was going to cry, which was weird because I thought I was totally over him! And i defiantly don’t want to be that jealous ex that only wants them back cuz they have a new girlfriend. I want him happy and I’m happy for him but my head is everywhere rn.

I then started to laugh at the photo cuz his new girlfriend looks exactly like me?!! So I’m sad, angry, amused, confused?!! I talked to my sister telling her I don’t know how i feel. She’s said I think I just miss the idea of having that special someone. She said I’d be over it. But it’s the next day and I’m depressed asf. I don’t want to go out. I can’t focus on anything because I’m constantly thinking about my ex. Idk what to do at this point. Do I still have feelings? Is this just a phase? I have no clue, but not even when I broke up with him was I heartbroken I was happy but now im “heartbroken”

I need some advice :/ or have any of you been in this situation?