Assalamu Alaikum
I have to agree with the above post. I honestly don't know what to advise you, and I believe this is due mostly because of how you are handling it with such patience and selflessness. May Allah reward you and grant you ease ameen.
I don't know what circumstances surrounded your husband at that time that he cheated on you, of course there is still no reason or excuse at all for him to do what he did, however if has repented and is showing great regret, asked for your forgiveness, and is willing to prove himself as a better husband, and you still love and care for him, then it may be better to just forgive and try to work together and heal from this. There is still the issue of the illegitimate child though, how is he handling it? Will he own up to the child being his or will he give it up to the other man? Is he still in contact with the other woman? Is the woman a muslim?
If he is not showing any regret and has not repented, then you should leave him because he is a fornicator and may repeat the same mistakes again.
Islamically, since the father of the illegitimate child does not know about what happened and his wife wants to keep it that way, then the child should be raised as if it were her husband's child, not your husband's.
You can read more about this
here and
here.
And as mentioned previously, try to seek out a knowledgeable third party who will be able to direct you towards the correct path inshallah. It will be difficult to try to conquer this all on your own unless you are really confident about your choice. Continue to pray and make du'a, may allah bless you ameen.
fi aman allah
w'salaam