I just learned that my idea for a Tear-apart stress relief doll already exists and is patented My version has 1 significant difference, can I legally produce and sell mine?
You have not provided enough information for your questions to be answered, but it is unlikely that you could provide enough information here for the question to be answered definitively.
An outline of the answer follows. Let's say the old (prior art reference) patented stress-relief doll has parts A, B, and C.
If your stress relief doll has parts A, B, and D, where your 1 significant difference is changing an element in the claim, i.e., C -> D, then you might be able to get a patent and make your stress-relief doll. Even if your invention is patentable over this reference, there may be more prior art which you haven't found yet - the USPTO examiners are highly-trained searchers, and may find things you didn't.
If your stress relief doll has parts A, B, C, and D, where your 1 significant difference is an improvement on the prior art patent, then even if your invention is patentable over the reference you cannot make, use, or sell your invention without taking a license from the patent owner of the prior art doll while the prior art patent is still in effect, even if you hold a patent for your own invention.
This is because a patent is not a right to make, use, or sell your invention, but rather a right to block others from making, using, or selling your invention. Similarly, if the other patent holder thought your patent was a great idea, they would not be able to copy your invention (legally) without taking a license from you.