I agree with Lovesmemore and also Blonde. There are some very spiteful responses on this thread. Par for the course. Some posters are able to have empathy and understanding and some posters just aren't, they want to lash out and hurt. Ignore them OP, there's some very good advice on this thread for you if you can disregard the vileness, it's always the same.
For whatever reason (and nobody here knows what it is), your OM is treating you very badly. That should be enough for you to realise that he is not the man you thought he was and that there is no point in your continuing to chase after him. In fact, if you do, he will have contempt for you.
You say that you're staying in your marriage because of your child. Is that all there is? Is there anything salvageable with your husband? Is there a reason that you and he are no longer intimate and have you talked to him about this? Sorry for all the questions but, I think it would be helpful for your to step outside this 'mess' and think about what it is you want. You know what you have - a husband and a child - and you know how you feel about them. What do you want to do next?
You need to disregard this OM in your plans now and in the future. No, you shouldn't be having an affair, it's never a good thing and it will mess up your thinking like nothing else can... and that's if you don't get caught. You say that you were caught out in a lie. Do you think your husband suspects? How would you feel if you were caught out having an affair? Would you be heartbroken to lose your marriage or relieved? If you can answer that question first, it would free you up to consider where you want to go from this point on. If you would feel relief then the best thing to do would be to suggest to your husband that you separate - or have an open marriage - and continue to parent your child.
If you think that you could repair things with your husband then the two of you need to talk about your marriage and what it means to you, what you want from it and find a way to make it work without other people entering your marriage.
I know you are upset about the other man. It's not love, it's 'limerence', look it up on Google. The intensity is a giveaway and it's not real. You might feel a bit better if you can read about what is happening in your head.
It's nobody's business to tell you how you feel about this man. If you feel that you are 'grieving' then grieve. Whatever it takes to make you accept that this, whatever it was, is over and that you will be moving on, either in your marriage or as a single woman - but without him. It's not as scary as it sounds.
You can post here, anybody is free to do that but, there are women who have been deeply hurt and it really would be crass to post about your affair in terms of what it means to you because I don't think anybody would want to join in that conversation. Post for advice about getting over him, sure - how to move on, great... there are some very wise posters here who will be able to help you.