How do I accept my situation ?

In other words: How do you accept changes? Sorry for the lengthy rambling ahead.

I'm 19. As of this year, I had broken up twice with 2 people, lost and cut contact with several friends, and my academics declined quite severely. Long story short: a lot of decisions were made during a very tumultuous state of mind. Admittedly I wasn't the most mature person in a lot of my former friendships, and I wished I had been a lot more proactive in discussing the problems in the relationship.

While I'm trying to make peace with the changes and keep my head up, I can't help but always return to the thoughts of 'What could have been had I not done this/that?', 'If only I had held on/been braver/more responsible' etc. I'm semi-aware of the lives of my estranged friends and those who I have cut contact with; they all seem to have pleasant lives, a good relationship, a decent job, a new group of friends.

I understand it should not be my business to pry or compare myself to them. But, I can't help feeling like I was their anchor to a fulfilling life. That I am a sore loser because I cut them off.

How do I accept my circumstances and move on?