First, let me say that I suffer from a chronic pain and fatigue disorder which makes living like a normal person impossible. I had intended to go to the evening Mass on Sunday but after doing the grocery shopping, going to a clothing store, and finding a garage sale along the way, I was totally exhausted when I got home at 3 PM. I ended up having to lay down from pain and fatigue. I admit this was due to poor planning on my part but I’m wondering if I should confess this now as a mortal sin or venial sin. (I did intend to go but got sidelined.)
Yes, fatigue can be a valid reason to miss Mass, especially when it is symptomatic of an illness or when it would make a person a dangerous driver. Given your personal circumstances, you may wish to rearrange your schedule so that you go to Mass in the mornings whenever possible, but you did not sin in this case.