I can't stop thinking about having sex with my parners mom.
Why would you even think about that?
Well, I haven't felt the warmth of a mature woman for a very long time. I'm lonely.
As long as those just remain thoughts I guess it's not terrible but don't let them get out of your control. Everyone has fantasies now and then but that doesn't mean it's okay to act on them. If you're really that lonely you should have a talk with your partner and let them know.
What I think is none of your business.
It would ruin your relationship with your partner if you kept having those fantasies. You need to sit and have a conversation with your partner about how you feel.