When you do find a saddle that fits you better, fear not yhe obnoxious lump on your inner thigh. Higher end saddles recess the stirrup bars but a more affordable one should not be overlooked. There is a fix in the form of Wintec Webbers. They're a steam lined stirrup leather design to remedy this very problem. Been using and loving mine for 3+ years, now. Posted via Mobile Device
Until you get another saddle (since this seems to be stemming the issue for you), I would definitely work on no stirrup work. Keep your calves on the horse, and be sure not to squeeze with your knees. Heals down (don't force them down though, just let your weight drop into your heal), and try to keep your toes from pointing out. You can also achieve this if you bring your stirrups up a notch. You can also bring your stirrups up on your western saddle (gasp.. I know! Lol) to work on your leg position until you are more comfortable in your current saddle, or get a new one ;)
Other than that, you have a great posting trot and I hope you enjoy your transition into the English discipline :)
When you push your leg forward while trying to keep your heels down, you are pushing your hips back. This exercise, which is good for causing laughter, will help you better understand how to position yourself. Have the horse stand square and balanced and stand up in your stirrups. Easy right? Except your bum is sticking out behind. Keeping your heels down, not hard, pull your hips forward and get your shoulder up until your are vertical and balanced. This is usually when the laughter starts. And keep your chin up, no letting your head tip forward. Now that you are balanced lower your hips. Stop. You're letting your bum stick out. I want you to try to sit on the pommel as you lower yourself. It won't happen but you need that mental image. You will use your hips and knees to lower your hips. If you're still vertical your hips will seat into the deepest part of the saddle and your ankles will be under your hips, correctly aligned.
I agree that this saddle is not ideal for you, and will not support a correct ear-shoulder-hip-heel line. However I would suggest that rather than riding without stirrups (which is certainly a good exercise, BUT can actually promote gripping of the knees and thighs, producing a swinging lower leg, until a rider already has a reasonably secure seat and quiet lower leg), try riding in 2 point. Shorten the stirrups a good 4 holes, stand up in them, rest your knuckles just in front of the horse's wither, keep your back straight, look up and stick your bum out!
If your lower leg swings forward, you'll fall backwards. If it swings back, you will fall forwards. I have yet to find an exercise better than riding in 2 point at walk, trot and canter, to correct an insecure lower leg, and a gripping knee/thigh.
Thanks everyone for the great tips! I already have 5 people showing me English saddles once the weather isnt horrible! Funny enough I can do two point in my western saddle without my legs moving at all, so ill try it once I shorten the stirrups!