Hi! It certainly sounds like something a urologist should look at but it can be hard to find good doctors, if u live in the US theres an article in the NYtimes that might give you some pointers in your search for one http://www.nytimes.com/2008/09/30/health/3 0find.html

If its hard to urinate when your not having an erection it might be something to do with the prostate and things in that area cause that and your description of feeling pressure in the head of the penis when erect sounds alot like the feeling you get when you squeez your prostate or its simply the tissue and skin of the penis that is tight and hasnt yet gotten used to the blood pressure but hopefully thats something that should go away on its own.
But find a specialist and dont give up,

Two things you could do until you find one is

1: always try to go to the bathroom often and before you go to sleep because even a semi-full bladder can be more than enough to start giving you a boner and

2: maybe try taking a beta-blocker(try beta-blockers first)or an antidepressant not to make you less depressed but to cause impotence there alot of drugs that cause impotence and prozac, zoloft etc they all have impotence as a side effect their also very easy to comeby just tell a doctor your feeling sad and they'll wright you a prescription on the spot, but that should be a last resort zoloft and other antidepressants all come with a higher suicide risk and never being able to have an erection might be very depressing.

Hope you find a good urologist, goodluck and remember Dont give up!

/Matty M