Should I forgive my friend his betrayal?

My best friend and I were in love with the same girl a few years ago, but she eventually decided to get into a relationship with me. He and I spoke about this, and I asked him if he would have a problem with it, but he said it was her decision. That relationship ended a few weeks ago, when I found out that he had an affair with her behind my back. This happened a few months ago when I was out of town. I broke up immediately because I felt betrayed but, when I confronted my friend, he said I should forgive him because he didn’t have a problem when I first began dating this girl. I’m not sure how the two things are related, but he has been calling me daily asking me to forgive him. I miss my friend, but also think what he did was unforgivable. Am I being unreasonable?
You’re right on two fronts: these things are not related in the slightest, and what he did was unforgivable. You were betrayed by your ex-girlfriend as well as your friend, both of whom ignored you, your feelings, and the respect they ought to have had for your respective relationships with them. Your friend may ask for forgiveness and giving it to him will always be your prerogative, of course, but you ended your relationship because your girlfriend cheated on you. Your friend was equally to blame because this wasn’t something she did alone. Go with your instinct, and whether you think you will be able to forget this, because not being able to forgive completely will only lead to resentment in the future.

I want to ask my girlfriend to marry me but don’t know if she thinks it’s too soon. Should I wait for a year?
This is a conversation you and your girlfriend alone can and should have. It is only by being honest that you can both understand where things currently stand.