If the people in the hearing (and perhaps even your lawyer) do not find you credible then there's nothing much that you personally can do about it, as I guess you have concluded so you are asking about getting another lawyer.

Have you considered asking your doctor to write a description of what happened with your blood vessel during surgery? And as much else as he knows? And send that to your lawyer?

Also, is this the kind of thing you could ask your therapist? I know she believes you, perhaps she can think of ways to convey information to your lawyer about your case in ways that SSDI will find credible? I know she isn't a disability expert but she may have some knowledge about people and communication strategies, etc.