Phone contract problem?

Right, so basically back in July I broke my iPhone 6, so we took it back to Carphone Warehouse, and they said they could give us an upgrade. All the upgrades were way too expensive so we went with a 6s, 32gb and 8gb of data and unlimited texts and calls. Worst mistake. It's pretty slow, crashes and turns off all the time, which I believe is because it's not a newer iPhone. Please don't come at me and tell me to get an Android it's not about that. It costed £80 upfront and £33 a month, totally not worth it for this phone. So I decided I want to end the contact, but it will be way to expensive. So is there anyway I can just end it without paying because it is literally draining my bank account for something that isn't even worth it. I know I got myself into this but I really need to get rid of this phone.

There's no way out of the contract.

The phone age is irrelevant; the general performance should be no different to the day they came out and it's faster than your previous iphone 6.
A 6S should work fine!

Problems are normally due to something going wrong during a system update, or running out of storage and corrupting itself - they are in reality a computer and a corrupt update or storage is just as possible with a phone or tablet as with a normal PC.

Back it up using itunes then do a "DFU Restore".
That's the equivalent of totally reformatting a PC and installing the operating system again.

Once it's finished, it restarts as if it were a brand new phone.
At that point you can restore your backup if you wish, or just use it as it is.

If it still does not work right - take it to an Apple store, there's something wrong with it and they should repair or replace it under warranty.

More info