My penis has shrunk over the years(not suddenly)I lost 3-4 inches I believe its my brain not enough serontonin

Over the years (not suddenly) my penis has shrunk. I know what your thinking a penis cant shrink will wrong mines has. In the past I had a big-penis but has shrunk 3-4 inches thats alot for just an imagination right. I heard a penis can shrink due to the brains, I can get my penis hard when aroused but cant erect, another thing is I havent had morning erections in like 5-6 years, sometime I wake up in the morning with a weird feeling in my head (I think that was suppose to be a morning-erection but the brain didnt have the fuel or shooting-blanks to do it, just my oppinion). When I do engage in intercourse I dont get an orgasm but I ejaculate (ejaculation is mainly water substance with a little or no semen). I believe the problem is due to my brain, I've been checked-out and everything is all right with me (heart, liver working just find) and im only 24-years old and my penis has been checked-out and it looks find. I believe its my brain because when I get hard my penis goes right back down, I have a hard time sleeping......I read about serontonin could this be what im lacking. I was circumsized late as a teen (age13) also i've gotten into a fight and took a heavy blow to my head as a teen also. I have a hard time sleeping as well. If you can please help me with whats going on to me, thanks