There is no standard solution. It normally takes time to find out what is wrong and to find a solution.
Try to find out what causes you erection problem. Negative thoughts? Fear of failure? Feeling guilty? Inappropriate circumstances for sex? Partner conflicts? Making love to a girl while you feel more attracted to guys? Fear of too much intimacy? Not enough physical stimulation? Feeling uncertain about your body? You have to be honest with yourself to find any answers.
Think about the first times you did not get an erection. Ask yourself questions: How did you feel when you started to make love? Did you desire sex? Did you want to have sex with that person at that time? What was happening in your life during that period? How was the relationship with your partner?
Also try to find out what is bothering you now about your relationship or about sex, apart from not getting erections.
Make a mental list of your prerequisites for good sex. Do you have to feel really comfortable with someone to have sex? Do you need the lights on or off? Do you need to shower beforehand? Do you need a quiet place where you are not disturbed? Do you need emotional intimacy before you can have sex? Do you need the presence of a fetish? What’s the minimum amount of time you need to get into it? Do you need to be relaxed? How much foreplay do you need? Make a list of prerequisites and ask yourself whether these were fulfilled the times you had sex and did not get an erection.

What you have to do depends on what is causing your problem. If you cannot solve the problem yourself go to a sex therapist. Don’t wait for years like so many guys do. The quicker you seek help the easier it is to solve the problem.