First I want to say  I think it was wrong of your counselor to suggest that you bring your partner to your hobby. A person needs to have their own things they enjoy separate from their partner because it helps to maintain individuality, and that was your sanctuary, so of course you were going to feel like he invaded it and build resentment, especially since he seems to have a much more outgoing personality (which is not better or worse in most cases but is worse when someone just wants to be unnoticed and enjoy what they’re doing). That was terrible advice. There are PLENTY of hobbies out there that you two could’ve done, so what should have been suggested is that you two try new things until you find a hobby you both enjoy doing together. If you ever do get back together… go try new things and let each of our favorite hobbies remain your favorite hobbies alone, without the other.

Anyway, so you say you began to act the way he was acting and couldn’t help yourself, right? This doesn’t make either of your behavior okay, but it should at least help you with the pain of feeling hurt by what he did, because when you experience the other side of things, it can help you to understand it. Perhaps he was also dealing with things internally that were making him feel angry, anxious, and resentful that were making him treat you in a way he shouldn’t have been… and these wouldn’t even have to be about you. Sometimes when we’re dealing with stressors that we can’t control, it affects all of our other relationships and that can cause us to treat the people closest to us badly, even when we’re not mad at them in particular. So use your memory of the monster you became to forgive both him and yourself for the pain you’d felt previously from him being the same monster to you so that you may release that pain.

With that said… I’m not saying you should forgive and get back with him. Forgive to release the pain you feel, but I also think you need to trust your gut on this one. It told you something was going on for 3 months and it was confirmed when the girl told you they’d been talking for 3 months. He may not have technically cheated physically, but there may be something emotional going on there and it sounds like he’d misled the girl as well, otherwise why would she leave and go home if he were taking her to meet her boyfriend? That makes no sense. I have plenty of guy friends and in that scenario, I would’ve reassured the girl myself that nothing was going on, we’re just friends, and he gave me a ride to see my boyfriend for whatever reason and then I’d go off to see my boyfriend rather than going home. So his story doesn’t check out in this case, and I think you have reason to feel that he was being deceitful and dishonest, at least partially. Maybe it’s true that he didn’t cheat, but the story he told you was not a story that made sense.

If you feel like you should block him and just walk away…. do that. You two have already done the breakup, said your goodbyes, and whether he wants to get back together or not, if you don’t… there is absolutely nothing wrong with just disappearing without explanation. You two aren’t together and you don’t owe him anything anymore. And a relationship with him won’t work anyway if you don’t feel like you can trust him and are still feeling extremely hurt by the past, so it may be better to just cut ties so that you can move on and heal.