My advice, don't go see your GP then. Make an appointment to see a urologist. Since you are able to have normal erections, it sounds like you don't have something psychological going on, but rather something physical. A urologist should be able to identify the cause of your issue and suggest a treatment.

I'd be surprised if marijuana was part of the problem, as my experience and others I've had sex with all think that marijuana heightens sensations. But I don't know if marijuana has been associated with loss of sensation in some people. You're right to stop all medications, at least to rule them out as causes. Then get to the specialist so you can find out what's really going on.