Not sure I fully agree. Lots of relationships break and end up back together for the better. It happened to one of her mates. Life also too short to give up on someone you love
But you did give up on someone you love OP. When you broke up with her for 12 weeks. You didn't try to work it through together, like partners do. You walked away.
And maybe she fell out of love with you, because of it. She might still have some feelings but you aren't the same man to her that she was dating originally with all the hope and trust that came naturally. You threw all that away.
"Lots of people" don't break up and get back together. Some people do. It's not that common, to then have a relationship last. It's very difficult to get over.
Just because a friend of hers got back together with an ex, it does not follow that she has to give you a second chance.
You are a different person to her now. One that hurt her deeply.
You'd be very lucky if she took you back, I think many people wouldn't. Shaky foundations don't make good relationships and it sounds like you still have issues going on.
You have told her you regret it and still have feelings for her. So anything after that is entirely at her pace and up to your ex girlfriend.
If that makes you anxious & not be able to sleep whilst you wait to see what she thinks, then it is from your own doing and something you will have to get over and not put pressure on her for. The same way she had to live with your decision and get over her upset at the end of your relationship.
You sound very young, yet you are 39. Usually one of the benefits of dating a person 8 years older is that they have a little more wisdom and stability. I'm not sure this applies here.
At 31 like your ex, I wouldn't be wasting my time with someone who is nearly 40 and was still an emotional mess who couldn't communicate nor pull together when it was needed.