Why do my palms go numb while I'm riding my mountainbike?

I have a low riser bar but I still have problems with pain in my palms. I have the thickest gel gloves I could find. I have added thick foam grips. My seat is the right height but I feel like I''m leaning forward too much. I don't want to raise the stem up higher, I already have some spacers to raise it up. Help me out please!!!