This tip applies in the summer, spring, winter and fall: Forgive yourself.
Every day, in small doses, before it can overwhelm you, take all that guilt you feel because you have missed a day of work, or not met a deadline, or had to bail on a plan with a friend, or had to miss an important family occasion, or had to miss the concert you were looking forward to (and had paid good money for tickets for), and on, and on, and on--and let it go.
In your rational mind, you know that sometimes plans don't work out.
In your rational mind, you know that you are not canceling on this plan to be vindictive, or because you do not love your kid, or because you do not think this project matters, or because you think money grows on trees. You are canceling because you have to.
In your rational mind, you know that you work extremely hard when you feel good, and you get a lot done. You know that you foreground your connection to family and friends when you can. You know that you do not waste money.
After all, you did not ask to have a headache disorder (or chronic illness, or whatever is plaguing you). You did not choose to feel bad. In fact, we all know you would choose not to have this thing.
So please: Listen to your rational mind. Be to yourself the kind and accepting person that you are to other people. Let things be what they are, not what you wish they were.