How long has this been going on? Are you referring to difficulties in sustaining an erection as the main problem, premature ejaculation or both?
I will list some things that I am familiar with that can cause this and then if you wish, I can move your question to the urology section.
1/ Performance anxiety, 2/ diabetes, 3/blood pressure medication, 4/if this is a new relationship the excitement level threshold is low, 5/ depression and medication that treats it.
Some things that you can do (depending on what you're comfortable with) are: using a penis ring, using hypnosis to manage this, having a medical exam to rule out the physiological factors, engaging in foreplay longer