I think you may just have to suck it up, you've bought a car and want to change it after less than a year but don't have the finances to do so, I mean you didn't really have the finances to afford it in the first place anyway from what you've said, so I'd just be grateful that you have a car at all. That may sound a bit harsh but I'm just laying it out there, you had £1500 to spend and are sat in a car worth presumably somewhere north of the low-ball £3500 offer you received. Had you not got support from parents you likely wouldn't have been in such a car to begin with.