Suggest Treatment For Erectile Dysfunction

Sir
I am 23 year old Indian guy. my sex life is quite ok. but now i am facing a problem. whenever i go to bed with my girlfriend & i try to enter my penis its loose tightness very quickly (without ejaculation). this problem comes on bed only. rest of time when i saw any passionate things, talking sexually with my girlfriend or wake up time its hardness is quite ok.
please help me i am so scared.