Your husband has violated your trust and the sanctity of the marriage big time. While I appreciate your comment that this has been "a very stressful time" for him, what about for you? You have been through 6 months of intensive chemotherapy!
You are confused at the present time because it has not yet sunk in. Yes, you are lost as to what to do. You are not just hurt. You are deeply hurt! Your husband has committed the one act that could totally destroy the trust between the two of you.
The road to restoration if after you've given yourself time to work this through is a 4 step process. It will not be easy and you may need counseling intervention.
The first step is for your husband to show genuine remorse. Not remorse because he is afraid of losing you but remorse because he realizes how devastating this has been on you and how dispicable his actions have been.
Step two is his desire to be forgiven. If he is is truly remorseful and is truly desiring your forgiveness he will also realize that he is not deserving of it but that it is a gift- a gift from you and he will receive it humbly.
The 3rd step is reconciliation. This is a difficult step and will take considerable time. It is the time time when you both begin trying to establish the trust that was destroyed. This can be done but it will take a lot of effort on both your parts. Again, counseling intervention may be very helpful. You will know reconciliation is taking place because the hurt and the anger and denial have waned. You have forgiven him and you want to move forward. You have moved well past the stunned/shocked phase. You may never forget the violation but you know in your heart your husband is worthy of your love once again. You are ready to move on.
The 4th and last step is restoration. Restoration does not always follow reconciliation. While you may have reconciled with your husband where there is now forgiveness and no more anger, you may come to the place where you decide you just cannot trust him as you once did and that is the straw that breaks the camel's back and that you no longer see a future between the two of you.
However, you sound like a strong woman who is prepared to go through the difficult healing process provided you see evidence of a truly remorseful and repentant husband.
What has happened to you cannot be trivialized and there are many a marriage that has died as a result of what he has done. However, there are many marriages that have been restored and become even stronger when both were willing to face the issue of infidelity head on.
It is not what your husband says but rather what he does from now on that will help save your marriage. By his actions you will be confident that not only is he genuinely sorry for breaking your heart but that he is resolved to never ever repeat this again.
Given all that you have gone through it speaks volumes that you are willing to give him a second chance. That speaks very highly of your character. I wish you well.