PLEASE HELP ME DOC!! Weak frustrating erections!

Hello to everyone, I am a 20 year old male. For the past few months I have been experiencing soft/weak erections. As I remember I used to have very stong erections 3 or 4 years ago, I could even hang 6 clothes hanger on my penis(not kidding). I still get erections tho but not as hard and as lengthy as before. When I have sexual intercourse I do get it, it takes quite a while to get it fired up and only lasts a few minutes. Let me add that I have been SMOKING CIGARETTES heavily for 4 years(never missed a day), I MASTURBATE 3-4 times a day since the age of 12(even after having sex), I AM OVERWEIGHT(red cross BMI is 35) and lastly I had an HIV scare for 6 month(thats how long I had to wait until I could definitely be sure I was negative, so anxiety and depression for 6 months).

My questions:
1. Does SMOKING, OVER MASTURBATING and BEING OVER WEIGHT contribute to this?
2. Is what I am experiencing now permanent?
3. Does circulation and me being a smoker play a part in this?
4. Does kegel exercise really effective?

DOC CAN YOU PLEASE HELP ME WITH MY DILEMMA. OR CAN ANYONE WITH SOUND SCIENTIFICAL AND MEDICAL KNOWLEDGE ENLIGHTEN ME ON WHY THIS IS HAPPENING TO ME AT SUCH A YOUNG AGE? OR ANY INFORMATION FOR THAT MATTER IS WOULD REALLY BE APPRECIATED. THANK YOU VERY MUCH