I Find Honesty and Trust Very Important In Any Relationship

Which is exactly why my relationship is "complicated" - I recently learned that my S/O has been lying to me about some major things since the beginning, and now the trust that I used to have in him is completely shattered. He tried to present things in a way that made it seem like I was overreacting or just misunderstood things - but he recently had to admit to having an entire other relationship he had lied to me about for months. He has kids whom I love with all my heart, or else I would have been out of this weeks ago when I learned that truth. I just can't figure out how to leave those little ones - I've never been so close to any children and can't imagine never hearing them call me mommy again - but I know I can't stay with their dad just so I can continue to be in their lives and love them. I had concerns about him lying to me a long time ago but ignored my instincts, and now I have to pay the price, but he's still lying to me and I just cannot live a life where I question every single thing said and done.