I Screwed Up Big Time

Me and my ex-wife got separated last month after years of struggling to make things work...we both had our reasons and decided it wasn’t worth fighting for anymore....She left shortly after that and I’ve been trying to pick up the pieces since then. A few weeks ago, I ran into a former classmate at a local diner, we chatted over drinks and eventually made our way back to her place...thinking only of getting laid. We ended up having sex in her bed on her living room couch, even doing it in the kitchen while she cooked dinner for both of us....It’s been eating me up inside ever since, I’m still married legally but separated practically, yet I cheated nonetheless. And with someone who isn’t exactly a stranger to the affair either...it was all so out-of-character and messy....Looking back on it now feels like one huge mistake that I can’t take back, no matter how much I want to....Anyone else ever find themselves in this situation?