You are stating in your question that you made assumptions about your payment that you don't actually know to be true or not. Frankly, your question cannot be answered without knowing the exact details.

I would suggest that you take the actual contract, then either read it carefully and add to your question, or take it to someone who is knowledgable and trustworthy, for example a lawyer, and let them check it, let them explain to you what it means, and if it is bad, how you get out of it.

Basically, you have purchased 25% of the condo for $40,000, and your parents bought 75% of the condo for another $115,000. We imagine for a moment that it wasn't you who lived in the condo, but some unrelated person paying rent.

You are paying $7,500 a year for tax and fees, plus $6,000 a year, so there is $13,500 leaving your wallet. If $15,500 a year was a reasonable rent, then the tax and fee would be paid out of that, there would be $8,000 left, of which you would get 25% = $2,000. If you were officially "renting" it, you would pay $15,500 a year, and get $2,000 back, again $13,500 leaving your wallet. So you are in exact the same situation financially as you would be if you paid $15,500 rent.

Question: Is $15,500 a year or $1,290 a month an appropriate rent for your condo? If a neighbour is renting his condo, is he or she paying $1,290 or more or less? Could you rent the same place for the same money? If $1,290 is the correct rent then you are fine. If the rent should be lower, then you are overpaying. If the rent should be higher, then you are making money.

Keep in mind that you will also be winning if rents go up in the future.