having an affair to even things out, won't make you feel better. short term mebbe, but afterwards, you'll still feel that empty, horrible feeling inside.
the fact is, however long she's been having that affair, she's basically grown sexually/mentally/emotionally away from you. Even if she wants to come back to you, b/c she didn't take you on this journey with her, she's somewhere where you are not.
This was unfair to you and prob unconscious act on her part.. a byproduct of her affair with another guy(s) and where she grew in a direction you didn't.
having said all that... there is still hope to get you and her back in the same place again. but only if both sides want it, and prob MUCH MORE helpful with a relationship therapist to help you both along in a constructive manner.
There are many reasons why people cheat on their partners... sometimes they want to end it, and don't know how. sometimes they just start off wanting excitement/self esteem boost knowing someone else wants them but then ends up going down a deep end where they alienate themselves/partner from each other, etc etc.
IF she still wants to, there may be hope yet for you two, but if you go down this road to cheat on her b/c you feel an unfair sense of justice/power... you'll effectively terminate any meaningful chance of reconciliation with her.
Good luck and I hope you make the better choice.
Btw, a better choice might be to just end the relationship, as well. Just don't cheat as a way to equalize how you feel; it'll damage you for any future relationships. My sympathies.