Why is my sexual stamina low?

Realize that being sexually aroused is natural, however, overdoing may deplete your strength and hinder you from gaining muscle. Get a new hobby that will keep your mind off this practice. You can also try exercising to help develop your body and eat foods that will boost your energy. In case this becomes difficult for you, talk to an older person that you trust or a counselor who can support and guide you through it. 

This can happen because of the sudden increase in blood pressure, muscle tension, or changes in hormones during the activity. To help lessen these headaches, you might try taking it slower, using techniques to relax, or turning your body to a different position. If the headaches still annoy you, it's a good idea to talk about this with a doctor.

It is safe to apply both a pull-out method and condom at the same time because it can prevent unwanted pregnancy as well as STIs. 

It seems that you had what's known as “pre-ejaculate” after your shower. This is a natural fluid that can be released before or after sperm. It usually occurs as a result of being turned on and doesn’t indicate any problems with health. 

Masturbation can't give you a sexually transmitted infe­ction (STI). Those come from bacteria or viruse­s shared during sex without protection. If you notice­ sores, liquid coming out, or pain down there, you might have­ an STI. See a doctor then, to ge­t checked and treate­d.