Really, you've had some fantastic advice. But I will say that the best bit is to sit down and actually talk with your parents. Ask them to give it to you how it is - don't fluff or sugarcoat it. Ask them flat out, ans tell them that while you love riding if there is more important things you are willing to cut down or quit for a bit (It sounds like if you have to you're willing). Offer to maybe go every other week instead, or cut lesson time in half. Riding is what you love but we all have to make sacrifices some times. If you've been riding that long taking some time off won't change anything. But the person you should be talking to is your mom and dad... Internet strangers CAN give you fantastic advice, but we don't know your family situation. They will tell you better than we ever can.
Another thing: Parents will do a lot to make their kids happy and hide struggles. When you talk to them, they may very well say "Oh, no, you can keep your lessons." Don't drop it at that. It IS what you want to hear, but it may not be the truth. Still offer to cut down to every other week. Me mature about it - I'm sure they will appreciate it immensely.
I give you this advice as a child who, when I was young, we had next to no money. I know how much of a financial burden was put on her - I didn't at the time, but I wish I could have realized (I was a bit younger than you are) because I know it would have helped her immensely but she would NEVER have told me herself just how bad things were getting.