You're talking the talk but not walking the walk. Taking full responsibility for your actions is not just breaking up with your girlfriend (that's the least you should be doing), it would include admitting to yourself that you could have acted differently, that it is your fault you let it go too far, that you were in actual fact not possessed by an alien and were in control of what you chose to do, etc.
So just saying "I take responsibility" and then saying "but I wasn't in full control of myself" (AKA it wasn't really MY fault...) or "but I didn't really love my GF anyway" (AKA I didn't really do anything wrong because I have stronger feelings for my ex) is not going to fly. Neither is a lot of history, guess what, I have a lot of history with my ex, too; did I agree to meet up over coffee to catch up? No.
If feelings aren't gone, what you don't do is
nurture them by hanging around that person for 6 hours. This is what you need to learn. This is not something that "just happened" and from which you can immediately exonerate yourself of guilt because your ex is "special", it is an important lesson on how to avoid temptation when you are in a relationship. And I get that me repeating this might sound condescending, but you sound like you genuinely do not understand the pitfalls of temptation and also incredibly young (despite your claims of "long history"
and this is exactly the kind of mistake people all make in their first few years of relationships/dating. And it's one of the things you learn by the time you have been around the merry go-round a few times.