Agree with all the posters saying be honest OP.
I found out my ex had been having an emotional/physical affair when I told him I was pregnant. I'd had suspicions previously and he'd followed 'the script' (if only I'd known about it then!) by calling me insecure, jealous, irrational etc.
He picked fights with me, was hyper critical and basically admitted later that he was too cowardly to finish things 
Meanwhile I was making excuses for his crappy behaviour and trying to be the 'perfect partner'...
He's now living with OW and although I hope it works out for them (all the pain and upheaval has to be worth something)
The lies were worse for me. Being made a fool of. The humiliation.
It's completly battered my self esteem and I'm now on ADs and in therapy.
Telling the truth won't be easy, but long term it will be better for all concerned.
As an aside, my dad had an affair with my mum's best friend. The fallout was immense - she had kids the same age and gender as me and my siblings. He moved in with them. They are still happily together 40-odd years later but none of us kids is married, 2 of us are estranged from the other parent of our kids, one is a commitment-phobe and another is in a marriage to someone that treats them like rubbish a lot of the time... Coincidence?
I'm not suggesting that my parents wouldn't have split, but the lies, deceit and hurt have stayed with me, and I think affected my ability to meet a decent partner.
Hopefully something I'll resolve in therapy.
Be honest OP, there will be fallout, but not nearly as bad as it could be if you're not.
I hope things work out for you.