Wow. I'm so sorry to hear of your predicament and I imagine it must be tough.
Now I have no idea why the two of you don't think you're right for each other as you haven't stated, but are you two certain that this couldn't work? Maybe time to grow into who you are supposed to be will help you either realise that you two can make it work with the growth under your belt or make you two realise you truly cannot.
Do you two stay in touch with each other? Maybe a no contact rule will help in letting go?
Other than that it's a choice. Choose to do other things, hang out with other people and maybe take up a hobby.

Thank you for your reply. The differences are in what we want long term. I have a 9 year old daughter who needs/wants a father and, while he has an 11 year old son and gets along with my daughter, he does not want to play the father role. I want to get married eventually but he just does not feel he could ever even live with somebody else. He needs his time and his space. He tried changing (spending more "family" time with us, staying the night at my house, etc.) but, it's not who he is and it started to take it's toll on us. We put off breaking up as long as we could because neither of us wanted to let the other go. When the relationship first started, we were on the same page, but then what I wanted changed; it wasn't his fault. We tried staying friends but it was too hard. We communicated out of necessity the first time in over a month last week, and it made me realize that that I am just as in love with him now as I was then and the progress I thought I had made no longer exists. I am stronger in the fact that I can draw the line and do what I know is best for me long-term (i.e. I told him I can't talk to him on a regular basis because it scares me and it hurts), but, right now, it is torment. I know it will pass and I believe that I will meet somebody more suited for me in the future, but I'm in a place where I'm angry (not at him, just in general) that it can't work and I want to throw a pity-party because it's just "not fair". Thank you so much for listening! I just feel like nobody understands.

I'm sorry to hear about the incompatibilities. It's always sad when two people love each other and things like that get in the way.
I understand the "not fair" part. Although my story is very different than yours, I get feeling like it's no fair, feeling angry and throwing a pity-party for myself.
I do understand the pain I really do. Losing love is not an easy thing to get over and deal with.
Keep strong and keep your head up and I'm sure it will get easier.

I'm sorry you're both going through this, too. I agree, it is utterly heartbreaking. Sometimes I think the best I'm ever going to be able to do is learn to live with it which I'm working on. People keep telling me that they don't think my ex is over me, either but there's no possibility of us ever being together again. He left because his feelings were getting to serious and it scared him. There's psychology that explains his behavior but knowing what's driving it doesn't make it easier or acceptable. The only advice is I can give is much the same as sinclair79 said - spend time with friends & family, stay busy and exercise seems to help me. Nothing strenuous but walking...that's a good stress reliever for me. In the first 5 weeks after he left, my dogs and I walked 119 miles. I have an app on my phone that tracks it which is how I know how much I walked. I had a lot of anxiety and shook all the time for the first 6 weeks. The only time I didn't notice the shaking was when I was walking. Broken hearts are brutal. Hugs to you both.

Ask yourself it he truly loved you. If you were right for each other, you would have worked towards the goal of staying together.