My old lady was similar last summer - at 27, had never worn a muzzle before and did not seem alllll that interested in learning how to use it (I also found that, after failing to get grass a number of times, she pretty much gave up trying at all - I tried making her "deal with it" for a week in hopes that something would click but she never figured it out).
Anyway, so I bought another muzzle (I buy the cheap $14 ones from horse.com and cut out the parts that cover the nostrils, creating a cheap "Easy-to-Breath" muzzle...Lacey's easy enough on it that I haven't had any issues with the cheap-ness part) and, using scissors, doubled the size of the hole in the bottom. She pretty much immediatly figured out how to eat through that (it did take me hand feeding her some grass, but she caught on fast). Her grass intake was restricted but not as much as it would be with the tiny hole. Once she stopped protesting having it put on, I switched her over the original tiny-hole muzzle.
For the first week, she had one day with the tiny hole, one day with the big hole, next with the tiny hole, etc (she started giving up again when I tried to just switch her "cold-turkey"). After a week of that, I was able to successfully switch her totally over to the tiny hole.
Now she has absolutely no issue eating with the tiny hole and her weight is finally at a really good place...it just took her a while. Grazing muzzles are sooooooo confusing, apparently.
[ETA - I'm sure she might have eventually figured it out but I was worried about her not eating for hours on end and being stressed because she couldn't do her favorite thing - eat! I also found that when I brought the grazing muzzle out for the first time this spring, it was useful for her to have an "re-intro" to it with the bigger holed muzzle, just fyi]