If the first relationship isn't solid, why are they still in it in the first place? Shouldn't they be out of the relationship to begin with? If that were the case, this situation would never happen.
But don't get me wrong. If a relationship isn't 100% where you want to be, that doesnt mean it has to end. You and your partner need to talk and confront the situation. Either get the relationship where it needs to be or both agree to move on
"Oh Lord it's hard to be humble, when you're perfect in every way. I can't wait to look in the mirror, cause I get better loking each day. To know me is to love me, I must be a hell of a man. Oh Lord it's hard to be humble, but I'm doing the best that I can." Mac Davis