Warning: Harsh answer following
Let me state some facts:
- You probably took a PhD you did not deserve ("worthless" was your original wording) to satisfy the grant requirements
- You were paid unusually lots of money for a PhD student, probably without having to do TAs but working exclusively on the grant aka your research. (those type of grants offer very high paychecks even for the PhD students, since they also have money for relocation, living expenses etc...). In EU, PhD students on such projects are paid as much as €2700 - €3500 per month for as much as 3 years => you probably were paid >€100k.
- You probably had a very big travel budget for conferences, with little administrative overhead (contrary to most PhD students who are on a very tight travel budget)
- You put your signature on falsified documents, probably lied in person to your grant reviews, did not resign or returned the money you falsely took when you discovered the fraud.
- Having received all that money and getting your PhD, you now have a guilty conscience and a change of heart.
- You want to do the "ethical" thing but without voluntarily returning all the money you received from the grant.
Somehow, I cannot see you as a victim here.
As far as I am concerned, consulting a lawyer and voluntarily returning all the money you illegally received from the grant (after consulting your lawyer) are the only viable solutions if you want to come clean.