Consider; If he wants a person who wants a monogamous relationship, is it really you he wants?
My advice for whatever that's worth is that you stop what you're doing and now. You can't hope for a good relationship if you aren't a good person. It sounds as if your boyfriend should have taken more notice of you when you levelled with him but that particular fuck up is for him to take responsibility. You, in my opinion need to take responsibility for your own part in this. If you care about this guy so much, why did you choose to cheat on him? It sounds to me like you care more about this farce of a relationship than you do his feelings. If you care for him and respect him then why are you behaving so selfishly? I don't know the details of this relationship, nor do I particularly want to. You don't need advice from anyone on here for this mess. You know full well that what you're doing could hurt you both irreparably Any relationship is no better than it is honest and open.
There is no easy way out of this. The damage is already done and there is more to come.
I hope you find the courage to better youself.