This sounds like it happened in a Latin Country. I had the same thing happen to me in just kept my long term girlfriend. I don't regret that decision one bit.
If these girls both know your fucking around theres a reasonably good chance they have or will too.
Your in a lose lose scenario. If you stay with the LTR she's going to be all over your ass to make sure you aren't cheating or she herself will subtlely get revenge.
With the short term girlfriend you probably had hardly built up her trust at this point and any trust you had was shattered. That would be a big uphill battle.
If these girls both know your fucking around theres a reasonably good chance they have or will too.
Your in a lose lose scenario. If you stay with the LTR she's going to be all over your ass to make sure you aren't cheating or she herself will subtlely get revenge.
With the short term girlfriend you probably had hardly built up her trust at this point and any trust you had was shattered. That would be a big uphill battle.