Scuzy, listen: having an affair is not a crime. It's perfectly understandable when your existing partner is a shit, as well. Please ignore any crap about how only wicked people have affairs. Maybe sometimes people who breach monogamy are acting a bit selfishly at the time BUT it's still not a crime. It's not illegal, nor is it ANYTHING LIKE AS BAD as abusing a partner for years and years.
Thing is, if two people are in an agreed monogamous relationship and one of them breaches the agreed boundaries of monogamy, OK, boohoo, it's a bit sad for the other one. But tough shit, no one dies of it.
HOWEVER that sad other one has two options: work through the issue or walk away. If the hurt partner chooses to stay in the relationship while spending years punishing the other partner by means of physical or verbal abuse, suicide threats, snooping, never letting it go then that 'hurt' partner is a shit who deserves to be cheated on and dumped (and you probably only went anywhere near anyone else because the partner was an abusive shitbag in the first place) and has NO RIGHT AT ALL to any moral high ground.
And (given that this thread is still in AIBU) anyone coming on here whining about the horrors of infidelity - if you are hurt by a partner's infidelity, that's sad. It may well be true that you weren't a bad person and didn't deserve it. BUT your options are: end the relationship and walk away with as much dignity as possible or (and this may take a while and involve counsellors though you have to put in the effort and the goodwill) aim to forgive and be happy again. However, if you react with violence, stalking, suicide threats, criminal damage, public humiliation etc, then you probably were an unstable controlling fuckhead and you probably did deserve it.