Just thought I'd add my story to the mix.
I'm only 17, and 140 and find it extremely difficult to mount from the ground. I was told that I needed to learn how to, but nothing makes me feel worse than that moment when I'm not sure if I'll make it or not. I think it is something everyone has trouble with but doesn't want to admit. I'm always going to use a mounting block. Better for your horse, your saddle and your self esteem.
I don't think everyone has a hard time mounting from the ground. When I was younger, I had no problem getting on any horse from the ground. Not even my 17.2 hand Trakehner but I had a friend that was my age, that was less physically fit and was heavier that had a hard time getting on a 15 hand Quarterhorse. I think that there are a lot of factors. Weight, flexibility, upper and lower body strength etc... People who have bad knees have a hard time. People who just have not done it much, it can be improved on with practice usually.
I have seen all kinds of funny "mounting misses" over the years. It isn't always older people or heavier people. I have seen some very funny misses with younger, thinner people who just didn't have the strength or coordination at first. Like everything worth doing in life, it takes time and practice to master the skill. Don't every give up.