What Causes Erectile Dysfunction?

Hi, I am 29 and have some erection problems. I have always had a small penis (usually under 3 inches hard and about 1 inch flaccid) but we have found positions that can sometimes work. I also have always had some trouble achieving and mainatining an erection ( I usually ejaculate semi-erect but not quickly). Lately however it is getting worse - on a morning sometimes my penis is firm but still only a maximum of 45 degrees. Whilst I can still get semi-erections sometimes this is getting less and less and most the time I can move my penis slightly but not get erect at all. Also when flaccid and sitting or laying on my side my penis is retracting into my body. Despite issues in the past we have managed on occassion to have sex with me semi-erect but I am not even hard or large enough for that anymore (I have large testicles and when I do ejaculate there is a lot). I am afraid things may only get worse.