While it is 20 years for him, it is new to you. It is very normal to be feeling what you feel. He has had 20 year to be remorseful for what he did. If you chose to R, know that it is a long and slow process. I personally am on the 5 year plan. I am at 4 years now and there are still some days that I can't hardly stand it. However, at other times things are much better. My husband has been remorseful however, that doesn't take the pain away. It does create some consistency that has helped me slowly develop some trust again.
Be patient with yourself. Expect the roller coaster. You do not have to make a decision now and you do now owe him a decision now. Take your time...decide what it best for you. When you decide, think it out and make sure it is not an emotional decision.
Hang in there...I promise it does get better.