The OP is in the U.K. And here it is not easy for a young teen to earn money. They are not allowed to babysit until 14, nor do a paper round.
The latter has strict rules like they cannot start delivering until,after 7 a.m. which would make them late for school in many cases
Dark Pearl, you are not ready for a horse of your own, certainly NOT a foal. The mare is a 'companion' why is that? What is wrong with her that she can no longer be ridden. Certainly she has one of many possible injuries. These might well be genetic.
As a child I yearned for a pony of my own. Unfortunately it was never going to happen. My parents couldn't afford it. So, I spent every waking moment I could at the riding school, working like crazy, learning as much as I could. As my sister also started riding my parents could only pay for her one week and me the next.
Back then I could work delivering newspapers. I did two rounds, both on the way to school - this meant I received double wages and also saved my bus fare. I paid two friends to do these rounds on a Saturday so I could be at the stables. Sundays the stables were closed. I would also return from the stables and go,straight to the beach in the summer, picking up litter. With any luck I would also pick up empty pop bottles and get the deposit back on them. Every penny went to me paying for extra lessons.
Of course I thought I was ready for a pony of my own, only afterwards did I realise that there was no way I was!
Ride as often as you can. See if you can help at the stables though I think that rules have changed on this restricting what a stable owner can and cannot allow when it comes to children 'helping'
Younstate that board is only £25 per week. What does this include? I doubt it includes feed and bedding. Not at that price. If it does then it certainly is NOT a high end stables.
I hope you follow up with riding but be realistic about it. Ride as many different ponies as you can, it is all experience that will take you further than if you only rode your own horse.