A slipping saddle doesn't really fit, I'd look to that first, it may be something else, but it's worth eliminating. And if someone comes along, says it's a good technical fit, but won't watch you ride or agree that it slipping is an issue, move on.
I don't like fixed girths either, and there are better ways of getting girths done up than going for a humane one. Girth extenders have two buckles, but if you cut off an old strrup leather about 6" from the buckle and punch some holes in it you can use it to get the first strap done up, when the horse stops blowing out you can get the second strap up and take the stirrup leather off, doing up the first strap properly.