I'm not sure there's any properly coherent way that I'll be able to explain this, so I'll just do so in bullet points:
-I fell head over heels in love with this guy (I'll call him C) who my friends and I are almost certain felt the same. (~Early 2019)
-I thought everything was going extremely well between us, even though I had to go away for 6 weeks or so (Jul 2019) - when I came back everything was still really good, and I was about to take the first proper steps with us. (Sep 2019)
-I then find out literally the day before I was going to tell C how I felt, that another girl (who he had shown no prior interest in) had asked C out and that they were now dating. (Oct 2019)
-This was obviously a very difficult grieving process for me which took many, many months and, although I've accepted that I still somewhat love C, I don't feel quite the same way that I used to.
-Now, just the other day (Dec 2019) I found out that C and his girlfriend broke up a couple of weeks ago because "she felt he didn't like her anymore" and she thought they should "be able to see other people".
-She now gives me the dirtiest looks and never speaks to me (we were kinda friends before) and I sort of feel guilty. But I can't help but be really happy and I feel awful. C and I are talking again and although we are both aware that the dynamic isn't the same as it was before, I want to give it another try. However, I don't know whether I'm doing this just to spite his ex, or because I want to feel the way he used to make me feel or because I actually want to be with him. There's no denying that I don't feel the way I did pre-heartbreak but something in me feels I might, but then again I'm scared that something like this will happen all over again (ie I get to a point with C where I want to tell him how I feel and someone comes and swoops him from me again).
No need to tell him how you feel. Just let this relationship develop (or not) and go from there.
There’s also no reason for you to feel bad or guilty about his ex. In fact, its a good time to distance yourself from her.
Go slow and don’t let outside elements pressure you.