Your actual question seems to be in the description, a very different one from the main one. You seem to be crying for help, instead of asking for a generality.
Your actual question seems to be: "what should I do if my first love still comes to me while he is married now, and this hinders me from moving on ?", instead of "is it true that we can't forget our first love?".
I will still answer the generality, then the question you seem to be asking for real.
Can we "forget" our first love: if you mean "fall out of love for good and move on completely": If the contact is broken, yes, it is absolutely possible. I did it, and many people around me did too.
Without breaking contact, I don't know.
About what seems to be your real question:
Basically: you're wasting precious time. Talk to him about it. Talk about it to other real people.
If he doesn't know what this is doing to you, he should know. Tell your friends what's happening (if they can keep it for themselves), have some emotional backup if you can, or a psychologist, anyone, and talk to him. Go meta.
If he does know what this is doing to you, he might only be trying to keep power over you, or keeping you up his sleeve in case his marriage doesn't work, or be minimising the issue, or feeling guilty to have let you down, or having pity for you because he senses your misery...
Anyhow, talk to him, and to other people about how you feel, what kind of emotional dilemma you are in. Real people and real open communication are the key here. This void created after he left can be changed, but it requires you to open up and dare to talk to other people. To find companionship with (possibly new) friends... try associations, church, fitness, what you feel drawn to. Don't focus on forgetting him, focus on building other relationships for now.
When you will have other people to care for, the question: "how does one forget about his/her first love" will be partially resolved already, and you will be able to find it on Quora when it is time for you to consider it... Good luck, you're not alone.