Dear Sue,
Everyone has given you such great advice. I just wanted to add that you need time to sort all of this through. I don't think it's good to make a rash decion when you're still hurt and angry.
One of my best freinds found out her husband was having an affair when she was pregnant with their second child. She went through Hell, but they were able to work it out (took a lot of time and counseling) and 19 year's later they are still together and happier than ever.
You've invested a lot of time/year's into your marriage. My prayers are that you decide to try to salvage it. If you try and it doesn't end up working out, you will know in your heart that you gave it your best. I am so sorry you are going through this...keep us posted.
Hugs,
Shelley