How you feel after cheating on your partner depends on two different things.
If you love him/her and if you don't.
If you love him/her, you will feel so bitter and angry with yourself and you may likely confess your atrocities to him/her. It'll hurt you that you did cheat and you will regret that act.
If you don't love him/her you will feel liberated, relieved and happy. It'll feel like a cage being loyal to your partner for so long and you would want to break free.