BEP - I know what it's like to have a bad time at school. Luckily for me my mum homeschooled me after we just moved around so much I wasn't getting any sort of consistent schooling. But when I got to Year 10 I had to do distance education. Look into what TAFE offers, they have courses that you can do that a very practical and job-oriented - otherwise there are bridging courses to uni that you can do (my sister is doing one now) and you can get into Open University. Higher education is waaaaay different to school in terms of classroom dynamics, because everyone is a lot more mature - and even more so at open universities as a lot of the students are mature-aged. My Mum's currently doing her Bachelor's through RMIT, and my Dad did his through Curtin - trust me, if my Dad can get through uni you definitely can!
It's hard to prioritise at 18, I know. You have dreams and visions of what you want to be, but for many of them you need to sort out your finances and independence first. I always had to pay board at my parents, from 16 onward, so it wasn't such a big transition to move out of home into a share house - but a well-paid job goes a long way to getting you approved on a rental application or as a share house member. If I were you I'd start doing it slowly - cut down on shows and start investing time and money in your education. Doesn't mean you have to give up your horses, just that you have to reduce your showing or put it off for a season or so. Once you have the education side sorted and the credit card paid off, start saving a little pot of money for a bond and month's rent in advance (although often in share houses they'll only ask 1-2 weeks' rent in advance on top of the bond). It might take a little while but you'll get there eventually. Meanwhile you will get your certificate and be able to get a better paid job. Or if you're going to uni you can even work for the uni - and their casual stuff is generally much
better paid than other jobs at entry level.
Once you've got your independence sorted, an education and a better-paid job you can start showing again
You're lucky that your parents are able to help support your horses, so you shouldn't have to sell them, and they can hopefully help you deal with anything unexpected while you're on your path to independence.