I get full erection.but when i start doing it with my partner, i get confused & erection goes away.what can i do to make it normal/fine :/?

Find the cause first: To assess if one's erections have weakened, he should visit a primary doctor, who can check for possible causes: stress, depression, diabetes, high blood pressure, low testosterone, artery disease, high cholesterol, smoking, older age, etc... Causes found can be treated to help make erections stronger, firmer, longer lasting. A urologist can evaluate. Meds like viagra, cialis, or Levitra (vardenafil) may help.