How can he just go on with his life after 20 years together like I never mattered to him?

I can't give you an answer about how he is handling this situation but all I can say is your worth is not attached to how much you mattered to him. In relationships we (myself included) tend to mesure our value to how much someone shows their care for us but we shouldn't. It is hard to not care about something like this but never forget that your importance is not mesured by anyone else's thoughts on you. We humans are scared of change by the very natural way our mind works but you matter. No matter what he or anyone thinks your emotions, feelings and personality matter.