In 2015 our little “pocket pitbull” mutt was diagnosed with heartworm (evidently contracted from where he came from in Louisiana, and his original test from the rescue was a false-negative).
High energy little dude, but he also needed mostly crate rest for weeks during/after the pretty rough heartworm treatment . Never outside off leash, and no major playtime with his “big sister”, our other dog. It was tough, but we patiently made it through, mj2, and you’ll be able to do the same. Like you, i also consider him my best pal, and we need to bite the bullet and do what’s in our friend’s best interests.
It definitely sucks but it will be temporary, and dogs are so awesome, they just live in the moment and focus on the positives of their lives: a great affectionate owner, good food and treats, and a warm cozy place to rest. The hard part is for us owners! Good luck!