Hi, I'm Heather, an attorney with 15 years experience and I'd like to assist for informational purposes.
When you agreed to cancel your contract and sent the cancellation letter, which you were instructed to do by your realtor, at that point (depending on what was in the letter), the seller did not have any further legal obligations to you.
The seller is now only contractually bound to the original buyers, and although the seller may have the option to cancel the contract rather than giving time extensions, if the seller wants to work with the original buyer and if the seller opts to give the time extensions that is his choice. On the other hand, the seller can send the original cancellation letter and then choose to work with you. Since you canceled your contract with the seller, it's not really up to you at this point.
If you really want the house, then it might be a good idea to do an hour's consult with an attorney to have them review the documents you have signed. He might be able to find something in the cancellation that works to your favor, but based on the facts you have stated, I don't see you with an option to compel the sellers to work with you.
Let me know if this has answered your question.