How can i sustain a erection better?

See your doctor: This problem can only be solved by seeing your doctor and being evaluated face-to-face. After a thorough examination, your doctor should be able to tell you what's wrong and what to do about it.

First you need to: Figure out of what may be creating any erectile issues. One thing that can help is understanding the quality of your morning erections. If you still have them regularly and they are quite "turgid"/stiff, that would suggest the "plumbing" is working well(from a vascular and neurologic perspective) and that there may be a stress/psychologic component to your issues.