Hi Thank you for asking HCM I have gone through your query.When you have normal sexual activity or presence of your partner and her fondling you might get a good erection.As your mind is exhausted with the masterbation and imaginary thoughts you are getting less hard erection.Considering Premature ejaculation:if you have it in normal sexual activity you can try following.You can do squeeze and stop technique.You can deviate your thought to something nonsexual when you about to near ejaculation.You can put on a condom or condom with anesthetic which will decrease the excessive stimulation on penis and there by prevent premature ejaculation.If all these not helping then you can approach an urologist or sex therapist or Psychiatrist for behavioral therapy and medicinal therapy like anti depressants.Hope this may help you. Let me know if you have any further query.