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I'm sorry to hear about your situation. What you need to know is that generally speaking the contract is going to control, and assuming the contract is good, that you don't have an exit clause, and that you can't come to an agreement with the buyer of the house, then if you refuse to sell, the buyer can sue you and seek "specific performance" (a court order that tells you to transfer the property as you contracted to do so).
There are really only three ways to "get out of selling"... First of all there's the contract itself. It's unlikely that there would be any exit clause, but it never hurts to look. Second would be with the consent of the buyer. If you can come to an agreement to cancel the contract with the buyer, you won't have to sell it. And finally it would be to do something to render the contract inoperative. This is actually the most difficult, as you would need to show that the buyer fraudulently induced you into the contract, or there was some other contractual defense that you could claim (duress, unconscionability, etc...) You wouldn't have a claim if you were mentally fit and couldn't show any "bad act" by the seller.
In short, unless you have the consent of the buyer, it's a very uphill battle to rescind a contract to sell. You'd have to show that the contract lets you cancel or that the contract was void from the beginning due to fraud, duress, or coercion. If you have any evidence of that, then you should take it to a real estate attorney to seek a "declaratory judgment" rendering the contract null. But otherwise (and I really hate to say this) the contract will be valid and binding, and the buyer can enforce it against you.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, ***** ***** luck to you!