Just a further thought, take the class 2 and then keep your old job, and just work a Saturday or Sunday at your leisure via an agency you will be quids in and then over time you can decide if you really want to do it full time, you might feel differently.
With regards to your office job, they are generally much harder to obtain and are generally more secure than driving lorries, my advice is to hang in there and look for a better one. Working in the office for a good organisation in a positive culture may change your views, plus you can still blow of the cobwebs in a truck whenever you want. Furthermore, you could do evening classes to better your situation
I have done trucking full time and office fulltime, when I mixed the two I appreciated both and got sick of neither.
Good luck and don’t rush into things