Girl- I've been EXACTLY where you are. 

Congratulations, you have just escaped an abusive relationship.

Do you know why he was upset your sister came? Because she can see through his gaslighting and manipulation. It is hard to see it when you're in it, and I'm positive that even though he knew he cheated on you, he was going to use your emotions against you to make you feel crazy about him "inseminating" his friend (what a completely ridiculous attempt at covering cheating, btw). 

I just wanted to comment on here to tell you that you should feel GREAT. You mention that you've "never felt less confident"... that's because that's how he wanted you to feel when you were with him. When people get trapped in abusive relationships, it's never because of their vices, but their virtues. Chances are you were too patient, too forgiving, too understanding. I'm sure he took advantage of those wonderful qualities, and after a while, starting using those things against you as well. In doing so, I'm sure you felt whittled away. Well, you're not. And he has no more power over you than what you gave him through the benefit of the doubt. But that doubt is taken away, and you are free!

In a couple of days, the air will smell sweeter, and you will breathe more easily. Fuck him. Don't let a creep like that dictate your self-worth. 

Please DM for any support from an internet stranger.