My bf of 3 years is cheating from last 1 year and is now going to marry her. I'm not able to leave him

Your boyfriend clearly does not value loyalty in this relationship nor does he value you, but you already know that. You were letting him take advantage of you and he could afford to do so as he knew this well too. You say that you don't want to stay with him but at the same time you're not able to let go of him as well.
First thing first, is this what you want? If yes, go ahead. If not, is it getting you any closer to what you eventually want? If yes, go ahead. If not, stop yourself from pursuing short term benefits and taking the easier road. It is difficult, but no one said it was going to be easy.
We sometimes stay in relationships that we are aware are no good for us for various reasons like low self esteem, childhood experiences while being raised, the need for shelter, a partner, to avoid the feeling of failure or fear of being alone/single. So it is not the person or the relationship that is keeping you from being able to leave it but your own issues.
What would help is for you to identify what is keeping you in this relationship (what needs of yours are getting fulfilled here) and what fears you need to be able to conquer to be able to come out of it and ways to do the same.
If these turn out to be some deep rooted issues that you find yourself struggling to deal with, it would help for you to reach out for support to a counsellor/therapist who would be able to work through them with you.