I think it's great that you realize that the disciplines are quite different and are willing to keep trying. A lot of people try going from Western to English, experience exactly what you have, and just decide that English is stupid and give up.
English will allow you so much contact and communication with your horse. Keep trying, you and your horse will probably enjoy it!
When you start out, you're dealing with something that can be quite difficult to overcome: muscle-memory. Having ridden Western for 15 years, your body instinctively shifts weight, balance, and reacts to your horse, without you really thinking about it. I'm pretty sure that your first time in an English saddle, your body was reacting in all those same ways it always has, and you didn't even realize it or want it to happen. You have to retrain your muscles and reactions by working on your balance and seat. I'm "retraining" a rider from Western to English right now, and she's having the time of her life learning to feel the horse. It goes both ways. I've ridden English 39 years. Put me in a Western saddle, and you'll bring me to tears.
-1- Have a sense of humor. Realize that you and your horse are doing something new. See it as an opportunity to have fun and learn together with your horse.
-2- Recognize that you will be re-learning things you thought you already knew (sound like you realize this).
-3- Take your stirrups off and get lunged at all gaits.
First, just sit there. Place your arms out to the sides like wings. Have someone tell you when you're "level" and not banking towards Hawaii. Most people aren't level when they think they are. Relax. Don't clamp your legs. Rather, get your heel under you, then move your legs around until you find the position where you have the most area of your leg contacting the horse and saddle. Try sitting on your pockets like a lot of Western riders do. Feel how little leg-contact you have. Now sit more on your pubic-bones and feel how much leg-contact you get. See your lower body as a wet bath-mat and let your legs wrap around the horse. Repeat several times until you find that "sweet-spot" where you're balanced with lots of contact.
-4- Now walk like that until you're confident.
-5-Now do it trotting. Sit the trot to begin with (Hopefully, your horse has a smooth trot). You still don't get to clamp with your legs. Just sit in the sweet-spot and have lots of leg-contact. When you're confident, post. Yes - without stirrups. You'll probably need to grab the mane to begin with, but eventually move up to keeping your arms out while you're doing it. Remember when you post to follow the movement of the outside shoulder, or both you and the horse will be off-balance (outside shoulder up = your butt up).
Be sure to do all this in both directions. For giggles, do each step with your eyes closed as well. Repeat and repeat - it will build your muscle-memory.
When you can post the trot with your arms out and your eyes closed, put those stirrups back on and ride! Oh, and you're going to get killer leg-muscles doing this. Stock up on the ibuprofen.
If you think it sounds impossible, here's some inspiration:
Enjoy the journey!