Depends on the sentence. It does depend but on it's own would be shouldn't have.
First, go see a doctor to rule out any illness or psychological issues. For many, practice is what is needed. Masturbate for practice. Slow down. Also, do exercises to build up your muscles to exert...
3-10 seconds maybe, but its not bad if its longer.
It would be nurses should.
Should have. The confusion arises when the conjunction "Should've" is used, which sounds very much like "Should of".