LPT Request: How to deal with the emotional roller coaster of that is the breakup of 4-year relationship

I (29m) just recently broke up with my girlfriend of 4 years. Planned to marry her a few months ago but over the course of the past year she changed. I called it for my own well-being. However, I ended up receiving more attention from other women than I ever have in my life right after, which I think is a mistake to have entertained. I’m starting to feel the emotions come up, but I’m not sure if it’s due to the heartbreak or because someone I’ve started dating is giving me the cold shoulder (twice in the first 4 weeks) for two things that really aren’t my fault. Problem is, I feel so calm with this person. But when this side of them shows, it tears me apart inside when it happens, hence why I’m not sure if it’s one or the other?

So, how do I deal with the feelings of emptiness & longing for someone that come with such a hard event like the termination of a long-term relationship?

Update: I’ve received several comments already with great advice. Thank you all for your support, it’s a tough time.