I would not want to start up again in life. Definitely do not want to go through this again. My regrets are social in nature. I never had a lot of friends. I didn't go out with the ones I had enough. I stayed home too much (I still do this, so I am in active regret). I didn't get to experience a lot of fun things people my age usually do.
Also, (and I don't really want this to be a regret, because I was happy with him for a long time, but I was also miserable often...) I held on to my last relationship/friendship for too long a time. I think part of me thought this would be my only shot at happiness so I refused to let go. I wonder what my life would have been like if I had walked away a long time ago.
But you live and learn. Hopefully, I don't make the same mistakes.