I realize that everything to be said likely HAS, as this thread is many posts in, and I honestly haven't time at this moment to read them all, but I wanted to make a few quick comments:
1) 45 is NOT OLD...You have MANY, many great years left in ya!
2) AMEN to you for following your dream...it sounds like you're doing GREAT; & REMEMBER, feeling "slow" to come along at a very challenging sport is NORMAL. Even lifelong riders feel that way when they take longer than they expected to manage a new riding skill/horse-training issue!
3) NO INSTRUCTOR in his/her right mind would be UPSET if it required YEARS of lessons to get you where you want to be as you are providing them with SECURE INCOME during tough economic times! ;0) Kind of being tounge-in-cheek, but it's something to think about each time you feel your trainer thinks you should "hurry up" & get it!!
4) I was a returning rider at age 38 after a solid 15 years out of saddle, & even with 15+ years of youth riding and intensive lessons as a kid, after 15 years off, it took me a good 18 lessons to even build the core strength/returned skill-set to safely start working on my canter/jumping etc...and I can TELL when what I'm doing "feels" right or wrong, as the body doesn't forget....thus, even with that "advantage" I'm still not much farther along than you are...and I only restarted at six years younger than you are!
I love that you're going for your dream! THAT KEEPS US YOUNG! Keep at it & don't let anything hinder you! EVERYONE here will support your endeavors 100%!!
(Now I'm going to make it a point to read up on how very redundant everything I've said was--Ha!!
BEST TO YOU!! B2H
P.s. I'm still a bit sore for a good 24° after EVERY RIDE, & I'm so flexible I can literally wrap my legs around my neck and stand on my hands. Females are OFTEN more flexible than males due to bodies needing to be able to bear children (though I've never had the pleasure!) so DON'T KNOCK YOURSELF for lack of flexibility ...I'm VERY muscular for a "chick" and I guarantee you have core strength 20xs better than mine regardless of how much I work out/ride...keep THAT in mind!!