I'm not going to claim to be an expert at either discipline, so please don't judge me too harshly for my broad opinions!
I ride both western and english with my horse and I love them both equally. I have been riding western for about 20 years, so it feels really natural to me. I love the deeper seat and the longer stirrups. English saddles are VERY different. Have you ever sat in one? There really isn't much to them. Even the deeper dressage saddles feel like a tiny strip of leather to a western rider.
When I started riding english, the first thing I noticed was how much more I felt my horse moving. It was really strange to feel his shoulders under my knees, it almost threw me off balance. Speaking of balance, that takes some practice to re-learn as well! The stirrups (irons) are smaller, the seat is slippery (unless you are wearing full seat breeches), and there is no pommel or horn to guard you from sliding forward.
Just start slow, find your seat, work on your balance, and you'll do great. Get used to the feel of the saddle and the "close contact" with your horse. It really is fun to ride english once you get over the initial shock! I really love riding english because of the way the saddle makes
me ride. I feel more balanced now, and more in tune with my horse's movements.