Brother, i got the same problem like you. I am at the age of 26 .. 180cm and 85kg. I don't know why sometimes when i know i could 100% erect .. but look at it, it just somewhere around 80 - 90%. I do get tired easily and always feel so sleepy. I used to have sex like 3 times a week until once a month.
Anyway, dont worry so much. Everything change when i started to get myself to exercise more. I spend 2 hours in gym to get myself sweat. Other than that, you can go for a jogging. Perhaps, twice a week. This will help you a lot.
Remember, nothing is impossible. You just need more exercise so you wont keep think of something that you shouldnt think and stop worrying. Cheers
