Two things you might try:
- cheap thing to try: buy cushion stirrup pads, I know Cashel used to make them but I have no clue what all is on the market now. You want things that either 'plug into' English stirrups, or velcro around (any) stirrups, with closed-cell foam to cushion where your foot sits in the iron. Make sure they do not cause the stirrup to be unsafe-ly small for your foot when they're on, although I have not had a problem with this myself. May work, may not, pretty cheap to try.
- more expensive to try, but maybe you could find someone who owns a pair you could borrow to see if they help: try a pair of WIDE-tread stirrup irons. (Wide may not be the right term, I mean that the portion your foot sits on is extra 'long' along the long axis of your foot, you know? so the tread has 2x or more the surface area of a normal stirrup). They used to be marketed for trail riders and for people with knee problems; I assume they are still sold somewhere, I'm just way out of the 'shopping for doodads' loop these days 
		
		
	
	
Both of these are sometimes quite helpful for people with knee, ankle or foot pain when riding, so would probably be worth a whirl for you, opportunity permitting.
Also, are you making sure to warm up and stretch, thoroughly and properly, before mounting. That can make a significant difference too.
Good luck,
Pat