Does the Prime Directive apply to all Federation Citizens, or only to members of Starfleet?

No. It is my understanding that while the Prime Directive is primarily pointed at commanders and officers of the starfleet, it is actually more a philosophy or broad goal, than a specific written set of rules.

Therefore, citizens, by extension, have to follow the Prime Directive even though they may not be in the service of starfleet.

You would be violating Federation laws.

The Prime Directive is part of the United Federation of Planets’ Articles of Federation (Chapter I, Article II, Paragraph VII), as well as the first on the list of Starfleet General Orders and Regulations. So long as you are within the jurisdiction of the Federation, it still applies.

IMHO, if, as you say, you resigned your Starfleet commission, I would think that you would also have to renounce your Federation-affiliated citizenship in order to be no longer under Federation jurisdiction.