My husband's personal moto for work, "fake it till you make it" comes to mind.
I also have had those hard days.
After my father in-law was diagnosed with COPD, he lost the ability to work on his two acres... for nearly 5 years.
When my hubby and I moved in to help, I ended up having to tear down a large section of land that was left alone for too long. Let's just say the 30ft by 20(ish) ft piece of land was so bad, the vines had traveled up the nearby telephone pole, and nearly reached the lines before I tore it down.
That's when I discovered poison sumac. (The hard way) a weekend job turned into a month. (That's just the cleanup!)
It got done, and the jungle is now a blueberry/blackberry patch. (No more vines! Never again)
If you drink protein powder the same day as the work, and stretch before, your muscles will handle the strain better.... and ibuprofen works wonders too!
I feel your pain!
Go job!