Forgiving a cheating ex who is now planning to marry one of my best friend?

Anonymous
We were childhood friends and were each other's first crush. We started dating when we were teenagers. She eventually broke up with me. Shortly after, I discovered she was cheating on me while we were together. When I confronted her, she managed to convince me that it was all my fault and that she was not to blame. At the time, I put her on a pedestal and I actually believed her. Over time, I realized that this was her way of being defensive about what she had done. That breakup really tore me apart and it took me years to recover.

Regardless, one of my best friend started dating her 2 years ago. We don't live in the same city anymore, so I don't see him as much and he hid the fact that he was dating her from me.

He announced it to me a few weeks ago. Today, I received an apology from my ex for her cheating. She told me she was in complete denial about what she had done to me, that she was a teenager at the time, bla bla bla (classical excuses). Anyways, an apology is an apology. My ex also announced me that they're thinking of getting married and that she doesn't want to be the reason why my friend and I part ways.

I don't really know what to do. To be honest, I am still very bitter at my ex for everything that happened, despite the (LATE) apology. On the other hand, it's been years since that happened. I don't wanna loose my friend over this and I cannot stay angry at her my entire life.

What do you think I should do?
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Edit: I forgot to mention that she did not cheat on me with my friend. She cheated on me with someone else (who I did cut off from my life)
Forgiving a cheating ex who is now planning to marry one of my best friend?
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