Erectile dysfunction Last few months.)

I m 31yrs old, unmarried. My weight is 87kg.height 5.9". i don't have prob like sugar n bp. Last few yrs i m in physical relationship with my gf n soon v r planning to get married. Last few months i m facing prob of erectile dysfunctional... i loose erection just after giving few strokes n suddenly penis size shrinks to it's smallest size. Gf tries to masturbate or oral sex on me but as it gets erect it shoots within few sex only. My gf is very upset n lot of argument r happing bcos of it. She is even thinking of cancelling marriage.. I m feeling shy to visit doc as i m not married also.gf is also not supporting me. Pls help.. (Few things would like to clear more -  * I m facing heat prob from childhood.bcos of heat prob i face prob like headache, mood gets off sweating prob etc during sex. Bcos of heat i always feel like going toilet n thus i loose erection.  If i bath bfor sex i stay for longer period.but prob is i can't bath each time before sex.)