Choose to forgive the damage even if you never forget the wound.
This is especially true of relationships. "But there is so much time invested," you say. If you cannot get over large amounts of time spent on an error that is clearly holding you back, I encourage you to forgive the mistake, forgive yourself for engaging in the mistake, then let it go! Otherwise you'll get trapped in a cycle of what was and not what is.
I do not like carrying anger in my heart and I do not like going to bed angry. If my choice has ever been to leave a friendship or a relationship, trust that I have prayed about it for a VERY long time; and usually after forgiving quite a bit of mistreatment. Friendships and relationships sometimes end so God can move you to grow. However, it is when people turn their backs on those who never walked out that hurts deeply. This is when you learn to grow tougher skin to accept things you cannot change. You have no idea how strong you are until you go through something designed to obliterate the core of your existence and the framework of your functionality, yet you somehow are left standing, victorious and undefeated. Just a year ago, I was nowhere near this invincible and empowered! All I can say is...keep going! You never know how close you are to becoming who you are destined to be!