I'm a 21 year old male, and im sometimes struggling to keep an erection during intercorse, it has been almost 2 months since the first time so i doubt it could be performance anxiety. since i've had proper and irregular erections, what's the reason?

Since you're capable of having full erections, I doubt organic problems or true impotence, but I do believe there is a strong component of anxiety including performance anxiety playing A big role here. If you still have concerns get evaluated at a men's sexual heath clinic or by a urologist.

In addition to the excellent advice from Dr. C., consider seeing a clinical psychologist or licensed sex therapist. In young healthy men, the situation is typically self-consciousness. Clinical hypnosis and self-hypnosis training can be useful also (www.asch.net). Peace and good health.