Should you stay in a relationship with a man who moves back in with his ex if he tells you it is for convenience and just until the two of you get a place together?

I wouldn't call you stupid ... just that the fact you love him and want to believe him and hope that's the way things will go down. Sorry to tell you hon, but he's playing head games. If he needed a place to stay for convenience he could have picked a buddy to stay with. He's using her and you! Don't feel too bad, most women meet at least one jerk in their life and most men meet at least one woman that isn't the greatest either. Move on!

self respect?

Yes, you are being stupid! If he can pay bills with her, he can pay bills with you if that's an issue. However, setting a standard of self respect that low is like having "Walk on me and my feelings anytime" tattooed on your forhead. Why don't you tell him that you are moving in with your ex and see how much self respect he has. How you let people treat you is exactly how they will always treat you. Dont put up with it and don't make demands, simply say good bye and walk away with grace and dignity.