Hi, K. I feel for you. Facing the truth is hard enough, let alone having someone you promised to spend your life with lie to you. I'm a firm believer in prayer. My stbx left me, but I wish I'd have left years ago. Let us know how you're doing and what you decide to do. If you do decide to leave, seek the advice of an attorney. If you move out of the home there can be ramifications in what you will get, not to mention possible issues with custody if children are involved.
My husband was in to women wrestling....and liked feet...of all things. One day I changed the security controls on the conputer, which limited the access to just everyday stuff. The computer blocked anything sexual. It was entertaining to hear his frustration. Tee, hee, hee. I, too, was hurt by his behavior. I would also find Sports Illustrated swimsuit issue next to the bed (if you know what I mean). Very demeaning. You'd think by his actions that he never got sex....quite the contrary. He was very demanding behind closed doors. The less he got it...the more he wanted it. The more he got it...the more he wanted it. He was never satisfied....so he went to the World Wide Web.
I believe that pornography IS a form of infidelity. BTW...You are NOT worthless. Pornography is HIS hangup..not yours.
I would definitely confront him and show him the history on the computer. Tell him that you know.
You might consider asking him to go to couples counseling. Tell him whether or not he's on the porn sites, there is a trust issue that needs to be worked through.
A little history though....Are there children in the picture? If so, how old? I know I caught my son as a seventh grader on bigboobs.com. Be sure it couldn't be anybody else before you approach him.