Lyris, I think it will be a good investment to read Wild Coyote's comment here. He focuses on acceptance of limits and tells that that isn't easy. I know, by own experience, how hard it can be. We want to do more, but our bodies rejects. Take your time to grieve your loss (of capacity because of pain) and at the same time try to work with both your thoughts and the small jobs.

I have found it helpful to have a routine for the day. In that routine I have "baked in" time to change my thoughts. There is scientific evidence that change in thoughts will create a change in emotions. You said: "I do the small goals, and the immediate affect is something like, "this is utterly pathetic, I should be doing more, normal people do this every day, I want to do more, why can't I do more, this isn't achieving anything, this is not enough"". Your thoughts are echoes from a past when you were able to do more. If you set aside some time each day to have an appointment with yourself, you can write down all the "not good enough thoughts" you remember and ask yourself :

"What is the evidence that my thought are true/not true?" May be you find some lines to write down about how pathetic you are ("truth" the way you see it). But when you have to answer about the evidence for "not true", may be you will see that to force a person with pain to do this or that is not a good thing to do. If so, you have your first evidence about that your thoughts are not true. May be you will form a new sentence, perhaps like this: "I am not pathetic, but it is sad that I with my diseases cannot do more" or my be "I am not pathetic, but full of anger because the loss of my former work capacity." (It's only you who can decide what you shall feel).

The question is the same as a CBT therapist will ask. It is meant to help you think over your situation in a new light. (A lot of scientific studies tells that it works). Other questions in the same road are: " What are the worst thing that could possibly happen? The best? Most realistic?" "What are the benefits of this thought? The costs?" ....

My "recipe" for how to cope better with physical constraints and feelings are 1) do the small things - they are steps to being able to do more as time goes, 2) look at your thoughts, 3) allow yourself to grieve (sad, anger ...), 4) work on your acceptance of your limitations and acceptance of yourself the way you are now..

If a person has an accident and has to learn to walk again, that person has to walk one step at a time until he can walk the best he can after the accident. May be he will have to use a crutch for the rest of his life, - give up former interests. The good news is that he may use some time to find new interests.

Point five: Be good to yourself.