It’s a tough one, nothing is more miserable than being at a job you don’t enjoy, especially when you have a family to provide for. I was in that situation once and was stuck in a contract. It did get better after a few weeks but it was crap.
I think you have two options in your situation:
1. Suck it up, care a little less at work so you’re less stressed about it. Leave at the point you don’t need to pay the training back.
2. Look for a job that’ll cover the training costs. £2500 is a lot of money, but not so much when looking for a new job.
Good luck, I hope it improves.
Moonhook