Wondering eyes and bad decisions

Hello girls. So please don’t judge me as I know what I’m doing is completely wrong and I never thought I would do such a thing but I am in my first year of university. I have been in a 3 year relationship (I’m 20 years old) however I have been meeting up with this guy which started off as innocent but last 3 times we’ve met sexual stuff has happened and last night we had sex but I just feel awful as with my boyfriend when I see him I just can’t help but feel bad. But why do I keep doing it? He is my first boyfriend that I lost my virginity too and before I came to university I only had eyes for him! Never in a million years did I think I would do something like this. I couldn’t imagine my life without him he’s my best friend and we do everything together it would be like losing my arm if I ended things. I’m just a right mess at the moment advice would be appreciated