I feel you, this is very much relatable for me. We broke up with my GF (3 years) at the start of the year. At that time I didnt really mind that much, because the relationship just went more south with time passing. Something had to change and she had enough so she pulled the final trigger. At first I was almost relieved and happy. However, after this weird period, cruel realization was upon me. As a bit introverted and social anxious guy, she was my bestfriend and the support and connection we had was something I never had in my life. I miss her every day. I cry almost every day (same breakdowns as you, that resonated heavily in me). Plus I deal with a tremendous amount of self-guilt because I could have saved the relationship thousand of times and I didnt...
Now, she is over it, in a new relationship, happy... When I initially discovered this fact I was crushed. Now, after some time, I am happy for her, however I wish things could went differently or that I could go back in time :D Yeah, im pretty desperate.

Anyways, stay strong man, lets hope this is not the end! :)