You cheated in an open relationship or you hadn't defined your boundaries and stepped outside of them? If you actually did cheat and sought to keep this from your partner then I would say it might be time to invest in the current relationship rather than adding another person to the mix even if it's only for one night. Cheating takes time to mend, especially if you really had no reason to cheat.
If you broke a boundary (say your partner wasn't entirely clear that he/she didn't approve of whatever you did but you didn't set out to conceal your activity) then this is a bit easier to deal with...though still painful as hell. If this is the case then redefine your boundaries, and this time go into detail. Be HONEST with yourself and with your partner.
Perhaps you could try a FFM 3some rather than the same sort that lead to heartache before? Give him some royal treatment, re-establish your commitment to each other, and maybe begin again.
It's not easy but it CAN work.
Honestly, if your sex life is boring then you really should work on that first as a couple. Offering a potential lover a SMOKIN' good time is much better than trying to use another person as a sex toy in the hopes it will make it all exciting again. It won't...
I wish you both luck but as the party who has been cheated on I have to say you have an uphill climb ahead of you. I'm available to talk if you want, I have been where you are.