Ask yourself if you can deal with it happening again.
And if u're gonna give it another go...make it clear to her that IF it happens again, it's over.
She needs to know that you are not willing to take this shit again and not cross the boundaries ever again if it's going to work out.

If u can find it in your heart to forgive her, and to work towards a better brighter future together, then by all means give it a go...after all, marriage isn't always easy. U have to work on it. But she has to be prepared to work on it too, i.e talk to you about her insecurities before sleeping with your best friend??!!... and not just take the easy way out again....