Can I be bound to a contract signed on my behalf by my parents as a minor now that I am 18?

It depends. People under 18 lack the "competence" (in the legal sense) to enter into legal contracts, until the disability (i.e, you turn 18) is removed. However, you can ratify a contract after turning 18 by continuing to perform under it without objection. An example would be that you go to a car dealer and buy a car when 16. You make the payments. You turn 18. You have second thoughts about the car but keep paying for a few more months - at that point, you've likely ratified the contract and are bound to keep on paying for it. However, if you return the car on your 18th birthday and say to the dealer, I'm not going to pay you anymore and here is your car back, then you can avoid the contract. Without offering specific legal advice, you may be able to avoid future performances on a contract such as this by providing prompt notice that you intend to terminate.