<human>: I feel I can't stop thinking about an ex (kind of boyfriend) and I need help deciding how I should go about it.

So here's the deal; I, a female, was in a casual, sexual relationship with this guy almost three years ago.It lasted only thelree or four months. I am now 21 and he is 20. I broke it off when I got back together with my ex. He seemed pretty hurt at the time and continued trying to talk to me.The problem is I haven't been able to stop thinking about him since. The even bigger problem is that he is now in a relationship and so am I.

I can't help but think I may have made a huge mistake to not be with him. I think about him just about every single day, even dream about him sometimes. That has to mean something right? Also to make the situation worse...he moved into the same apartment complex as me, he lives just down the hall from me and my boyfriend. We live in a small town. I see him every once in awhile in the hallway and he makes brief small talk. That only makes me want more. What do I do? I'm going crazy!

Can you succinctly summarize the text in 1-3 sentences?
<bot>: I feel I can't stop thinking about an ex (kind of boyfriend) and I need help deciding how I should go about it.