I wish I had your strength to be honest with myself like this about 8 years ago..
You can't fake something that isn't there and I truly believe that staying with someone just out of fear of hurting them can only lead to more unhappiness. His heart will heal and eventually he'll probably come to realise that although you have caused sadness in him now - you've actually spared him deeper heart ache later down the track.
It can't be easy for either of you. I hope you both start to smile again soon.