There seem to be two separate issues here which, because of the stress you are under, have become fused together in your mind.
- Your financial struggle to pay for the school and how that makes you feel via a vis your own life choices. This is being compounded by what everyone has identified as his entirely standard teen attitude. It is horrendous living under that kind of pressure, but as pp have said - it was your choice and sadly he is not under any obligation to make you feel better about it (however lovely that would be)
- His educational achievements. Most independent schools would have come to you and flagged any issues which they felt might result in below par exam results. Doesn’t sound as though that’s the case here. You may not see him working as you would like/expect, but if the school haven’t raised this as an issue with you then it is quite possible he is doing well and will be absolutely fine come exam time. He may even be really good at exams and surprise you/pull socks up after mocks etc. Essentially, his performance is really between him and the school at this stage. It is clearly adding to your stress to engage with it, youre paying for his education, so ask school questions about his performance and leave it in their court.
Honestly, you’re creating a conflict where there doesn’t necessarily need to be one, by getting in a panic about exams which, as he won’t engage with your help, you can’t actually help him with.
Try to make your peace with the choice to send him private, and just see it as an investment in his future which you/he may not see direct rewards from immediately but will hopefully have a positive impact on his future (not because it was private school, but because it was a positive school experience).
Stop undercutting whatever is good about that experience by nagging and effectively telling him he is undeserving of it - this will only impact his well-being and your relationship. It will have zero effect on improving his academic outcomes.
Focus whatever energy you have first on trying to improve your own wellbeing (you sound totally burnt out and in need of being kinder to yourself).
Ask DP to help, create a set time weekly to do something which might positively impact your well-being. Eg. Sunday afternoons you go for a three hour walk/cinema trip/swimming/coffee with a friend. While you’re out he and his dad do one useful thing (make a meal/run some laundry/clean the bathroom). Let your DP manage this and organise it/reward him within that time as he sees fit. Just remove yourself from parts of this equation and try to refocus your energy on you prod using yourself with what you need emotionally, so there is less burden on the feeling that if he doesn’t keep his end up he is letting you down and impacting you.
I know it’s really hard - burn out is real. If you’re feeling this bad I do think you seriously need to disengage from your DS and his schooling and instead engage with what you can do to feel better in yourself. You can just say - “I’m done nagging you, I’m here to help if you want it, otherwise it’s between you and school and I’m going to use my energy on myself!” That models a much better situation for him (and for your younger child).