Sounds to me like you have a major hang-up about this. Why does it weigh on your psyche so heavily? It should not.
You should do your honorable best to pay whenever you feel like you should…but if they INSIST, then
let them. What's the big deal? If they insist on paying all the time then obviously they have some insecurity or other issues they are dealing with. Sounds like you may have the same.
Don't worry about trying to figure out how you are going to force your way into paying for them, worry about why you feel so compelled to pay for them. If you were being a bum and always finding ways not to pay then it'd be different. Then you would (or should) be feeling some guilt. That would make sense. But if you've honestly tried to pick up the tab on multiple occasions and then your friends insist they pay, then you should be ok with that. They obviously want to pay and it makes them feel better about themselves, then fine, be a friend and let them.
Figure out your hang-up with why you feel you
need pay. Work through it, fix it, and get over it. Then when successful you can share what you learned with your friends…maybe they can then take your advice and get over their own hang-ups. Then they'll happily start letting you pay.