ok first thing will be to get your stirrups at an appropriate length. What type of english saddle are you riding in? In a dressage saddle, the bottom of your stirrups should bump against your ankle bone when you allow your legs to stretch down out of the stirrups. Any longer and you will not be able to ride with the heel as the lowest point of your body and will be tipped forward over the front of the horse and forced to grip with your knees. You can start with them a little shorter though, but not over done.
With your stirrups shorter, you'll find it easier to sink your heel down and your toe will come up. If you are riding on your toes, it will simply tip you forward all the time and you'll lose balance, it's like fighting an uphill battle!
A good exercise to do is standing in your stirrups without rising at all. So stand up out of the saddle, with your crutch of the pommel, sink your weight into your heels and stay upright just at a halt. You can hold onto the saddle cloth with a finger if you like when you're starting out. Once you can stand comfortably in your stirrups, maintaining a straight back and not having to hold onto the saddle cloth or lose your balance, start doing the same thing in walk.
Again, same deal until you can stand in walk, then progress to trot. When you can stand up in trot, you will find you will have a million times better balance on the horse! Your rising will improve dramatically as you sound to just be having balance issues :)
Key points are :
-shorten your stirrups to a comfortable length in which you can easily weight your heels rather than your toes without having to grip.
- learn to stand in the stirrups to improve your balance and allow you to find your centre of gravity. When you can stand without having to hold on, or without getting the wobbles, you are in perfect balance. Just keep that same feeling while you're doing rising trot and you'll be just fine.
- Practice practice and practice. Avoid slamming down on your horse's back, if you feel that you're loosing balance and you're going to fall back, hold onto the saddle of a chunk of mane so you land lightly