Want to get new iPhone but have 18 months left on current contract?

Currently have Sony Xperia Z3 Compact, which I thought looked great. Turns out isn't as good as I thought it would be and moving over to an android phone from iPhone was bad idea. Now I want to go back to iPhone.

Any idea how I can do this without having to pay a lot of money to get the new iPhone on my contract?

You are stuck, thats why I tell people don't make a move you regret because now you are stuck with that for another year and half. The only thing you can do now is buy it unlocked with no contract deal, the phone company doesn't care what you use but you won't get it with contract price.

Go to a different carrier. Sometimes they'll pay your early termination/ cancellation fee for you. You can also pay your phone off in payments. I think ATT, Verizon, and T-mobile all have plans where you can do that but you might have to change your monthly plan. Your best bet is to contact your carrier company and ask them what you can do. If you haven't had it for more than a week, then they might even let you switch but i don't know I've never had to do that.