Problems related to sex. My erections?

I am obese 24 year old with 91kg and height 180cm. I have erections problems. When i tried to have intercourse my erection got away. When i lied on my back i had erections but when i turn out to try have a missionary position my erections got away . Also when i stand its hard to maintain erections. When i lie on back or when i sit i get better erections than trying to stand or in a missionary position. Why is it happening ?  I also have habit of regular masterbution.I check my testosterone level and they are normal in range.I also don't have morning erections. To keep the erections its hard and i feel a pressure in my stomach so that i can have erections. Why i need to apply some intra pressure in stomach (abdominal) to have my erections?