Join Date: Oct 2009
Location: Just south of sanity
You shouldn't ride with longer stirrups until you have a good, deep, solid seat.
Believe it or not, the shorter the stirrups, the easier it is on you if you're a beginner.
As a beginner, you need a shorter stirrup length. Your instructor should know where to put them to achieve maximum effect.
Dressage riders have longer stirrups, and the rest of us ride shorter because of our specific disciplines. There's no right or wrong on stirrup length unless you're showing competitively, as long as you have control of your seat and legs.
Your saddle's not girthed tightly enough or not placed properly if it's sliding when you attempt to mount. My saddle stays in place.
You might want to consider using a mounting block if you're having trouble mounting using an English saddle. Never grab the cantle on an English; it won't help you.
When I mount, I hold the reins and a bit of mane hair in my left hand, and my right helps me balance as I swing across the top of the saddle. Once I'm settled, my right hand goes to my right rein.