Oh OP, my heart goes out to you, what a terrible shock you've had.
I agree with everyone else on the thread, it's not anything you've done, this is all on him, please don't think that if only you were a,b,c (whatever) he wouldn't have cheated on you, because he would have done, because it's not about you it's about him and his insecurities and massive sense of entitlement. Look at it this way, even the most beautiful women in the world get cheated on (thinking of Jerry Hall and Mick Jagger here!).
I think that after 26 years together your sense of self is very much enmeshed with his, understandably, and you have spent a good portion of your life thinking about him and his feelings instead of your own.
If you can afford to see a therapist, I cannot recommend it highly enough. Meanwhile, start thinking about yourself more, what do you want from life for instance?
Try to stop thinking about him and what he's done, that's your past and knowing all the details won't actually help you heal, they will just create more wounds.
Think of it this way, whenever you start obsessing over the details of what he's done, you are giving him power over you. You don't want him to have any power do you? Take back control over your own thoughts and revel in your own power!
I imagine your self esteem will have suffered, so start building it back up. Make a list of everything you've done in your life that you're proud of and read it back every day. Remind yourself of who you are.
Make a list of every compliment you receive and read that back too, remind yourself of how other people see you and realise that it's only him who was stupid enough to not see the real you and how much you're worth. And remind yourself that you are worth sooooo much more than a lying, cheating, gas lighting scumbag!