Honestly, rebuilding trust after it’s been broken is one of the hardest things to do.
I think what you should do is explain to her that you were just scared she might leave you, and that’s why you didn’t tell her the truth. If she gives you another chance, make sure you take care of everything that needs to be fixed regarding your marriage with your ex-wife.
And from now on, you really need to be transparent with her about everything. You’re partners, and you should be honest with each other. Hopefully, you’ve learned from the mistake you made.