This situation didn't change anything about your character or his, it just illuminated what was already there. That he is a cheater, and that you are a really caring generous person. Cheaters cheat, so even if you hadn't discovered it this time he'd be cheating anyway. Not your fuck up. Everything you did was with a warm and generous heart, and I'm so sorry your partner is such a giant jerk and disappointment, you deserve much better.