Rainyday, sorry you had to find out, but at least now your know and aren't guessing and playing mind games. It doesn't help how I'm sure you're feeling, but at least you know.
First, I take it you're not married as you referred to him as your partner. Also not sure how long you've been together. If you're not married, you really need to ask yourself "Is this the person I want to be with". If he broke your trust, confidence, lied to you, deep down, you know. Let me ask another way. If you had a friend in a similar circumstance, or maybe even a daughter, what advice would you give? Personally, and it is a simple persons opinion, I'd be giving him a lot more than a "little" space. I mean a lot, like, "I deserve better than this" kind of space.
As far as confronting him. Perhaps start out by saying how much the relationship and he means (or meant) you know it wasn't "right", but was compelled to. You had suspicions, he knew you had suspicions... You checked and your suspicions were confirmed. If he denies, call him out on it. If he wants to keep the relationship alive with you, be prepared for his "rationalization" of his cheating. You were distant, cold, he didn't feel loved, she came on to him, he was weak, it wasn't his fault, it was your fault... the list could go on and on. Then, YOU HAVE to be prepared with what YOU, YOU, YOU want and expect. If you and he want the relationship to continue, YOU set the demands.
You'll receive a lot better advice that what I can give from others here. Much wiser than I. However, I do have two daughters. One young adult, the other late high school. If either of them were in a similar situation, I would so try to convince them that there are other men, better men, who worship their partner and would NEVER do anything to hurt them, emotionally or otherwise. They deserve better, as you do.
I know it's hard when you love someone. Please take time to thing what you want for yourself and make that happen-whether it's with this person or not. Best wishes!