Reconciling... Should I come clean?

My ex wife and I are talking about getting back together. We've been apart for almost a year, but we have a kid together.
She had a boyfriend for a while and told me they had sex, because it had happened one time when the kid was there with her and the kid told me he heard something. I don't know if she has sex with other men. She has asked me a few times if I have have had sex while we were apart and I lied, saying that I had not. I have had three one-night stands, all with protection.

I think she believes me and we talked about not discussing the past year, and instead starting with a clean slate.
However, I am not sure if I should tell her about having had sex. We're probably going to see a therapist and I really don't want to lie in front of the therapist either. I would rather just start fresh, with a clean slate, and leave the past in the past.

But...

If I tell her now, after lying about it, it might ruin everything.
If I don't tell her and she finds out (unlikely, but possible), it will really ruin everything.
If I don't tell her and she never finds out, there is a good chance that we'll reconcile and our family will be together again, but I'll have to carry that to my grave and that's a long way away (I hope).

So, I'm looking for advice.

Should I tell her now and risk losing everything or should I keep quiet, hope she never finds out, and try to move forward with reconciling?

This sounds EXACTLY like my ex. We have a child together, she had been seeing someone for a long time, I had a few flings, we discussed BRUTAL honesty and told each other everything. Right down to the last detail. We made it a month before the pressure of what she told me collapsed me and I freaked out on her because of resentment. We are now broken up for good; she is seeing someone else as I am too.

If I could do it over again I would keep my mouth shut and ask her to do the same; not lying but just not telling each other about the past year.
I feel we would still be together if I just didnt know.

Ignorance is bliss. Good luck, I mean it.

You shouldn't feel guilty. You've both agreed that the slate is clean and you're not discussing it. Guilt is a useless emotion; you'll find life much more enjoyable if you let the end justify the means. Where is the practicality in telling her?