If the marriage was already on a downward spiral then, there is more to the story than cheating. Marriage is like a business, success depends on how much effort you put into it. If you are not servicing your clients correctly, they will go somewhere else.
Forgive or not to forgive depends on a few things :
Is she willing to come back and end the relationship?
Was it is partly your fault? (mostly yes)
Do you have children?
Can you afford not to forgive?
Why would you want to forgive?