I agree, look up some good exercises to do and practice them to help you with strengthening. Any kind of cross-training might help you as well. If you don't have access to gym equipment, maybe you could try running or biking? I'm not a fitness expert, but I am tall like you. I used to never be able to get up without a mounting block. It was so frustrating. I don't know if you are riding english or western, but if you are riding english, is there any way you could get access to a western saddle? If you can practice mounting with a western saddle and using the horn for support (that's how I learned how to mount easier) that would be awesome. Practice that for a little while, getting on and off with the western saddle (if possible for you). Then switch to english. If you don't have access to western equipment or if your horse will not accept a western saddle don't worry. Keep practicing your strengthening. You could maybe do some lunges and stuff on your stairs, practice putting your left leg up on the step and pushing yourself up. Try it with your right leg to have an even workout (it's the same thing as teaching your horse leads: you want to learn both ways). Like I said, I'm no fitness expert. But I hope that I am able to offer some advice. Also, when you are mounting, grab the horse's mane if possible and the back of the saddle, not the front of the saddle and the back, you don't want to lean on the saddle that much and have it slide around. My saddle fitter showed me a great way to mount as well. Stand next to your horse's neck, facing the same direction as your horse. Then turn around and face directly next to your horse but backwards. Put your leg in your stirrup. Then grab mane and the back of the saddle. Hop 1, 2, 3 inching closer to the saddle each time. Then hoist yourself up. I hope that some of my advice works for you, but regardless I wish you luck and I hope it goes well for ya! Don't get discouraged. Keep trying!
God bless, englishaqh (: