A Mistake I Can't Undo

I broke up with Alex a month ago after two years of dating. We were both exhausted and needed space to figure out who we are now that we're not together anymore. But last night, I couldn't sleep, so I went to my friend's house for some company. It was supposed to be just drinks and maybe a game or two, but things got heated fast. She's been there for me through the breakup and understands how I feel, so when our hands started moving in ways they shouldn't have, it felt like a natural progression. We were both adults agreeing on this, but now that everything is out in the open, I can't help feeling guilty about it. It was a mistake made under stress and pain. Is what we did normal, or am I a terrible person for taking advantage of someone's kindness?