How do you know when to let go

After the hell that was last week I got in touch with the MSPCA. They gave me a few options. I could:
1. Try to figure out what was wrong with him, and keep him
2. Write up an ad, they would put it on their website, and he would stay with me until he found another adopter
3. I could return him to the MSPCA, and if he couldn't get adopted/fostered out/treated they would have to euthanize him

I went up to the barn the day after and Finn was trotting and cantering around, rearing up with one of his buddies, and playing halter tag. When he saw me pull up to the barn, he stopped, and trotted over to the fence from the other side of the paddock, ready to come in. He licked my jacket, pulled the zipper down, and searched my pocket for the treat he knew was there. As I was leading him in he put his chin on my shoulder and nuzzled into my neck. So, of course, I called the vet. The vet was out yesterday and spent 3 hours with us. First she watched him trot in both directions on the lunge. She noticed a little off-ness in the right front, which I wasn't noticing. Then vet's assistant trotted him out on pavement, and lunged him in the driveway. She still noticed the slight off-ness, now in both fronts.  She flexed his fronts, and it was still very slight. She decided to do a heel block on right front. He lunged 100% after heel block, and I definitely noticed an improvement in his stride after the block, which also made me noticed the bit of hesitation on the other side. It wasn't like he was lame, just a bit short strided, a little more careful to put his hoof down. Then we flexed the hinds, and she said eventually (years down the road) he would need his hocks done, but it wasn't a concern now. She checked his back, and he was SORE. Basically from his withers to his hips. She believes it's muscle related, and says it seems like he pulled something in the paddock, or, from all the recent training and learning to move and use himself correctly probably for the first time in his life he is most likely sore from that too. Before doing anything to his back she wanted to take x-ray's of his fronts. His x-rays look fantastic, and new farrier has made such drastic improvements in his hooves and angles. His soles have also increased a ton in thickness. Vet believes he is exhibiting a bit of heel soreness because his toe is still a bit long, and she wants farrier to take off even more toe, and set the shoe/packing back further on the heel, which should make him more comfortable. Farrier and vet are going to work together to get his fronts even better. Vet then recommended back injections, while he was sedated, and I was on board. Her assistant prepped him, and doped him up a bit more. Vet started trying to inject him and Finn said NO in a big way, popping up, kicking out, swishing his tail, and bending all her needles. His back started twitching. Unfortunately he is so back sore and tight right now that we are going to have to wait to do the injections. She put him on high doses of Robaxin and an NSAID for 10 days, she will come back next week for acupuncture, and then the following week we will try the injections again.
      She does not think there is anything terribly wrong with him, other than some super tight muscles that need some help. She feels that all of his behavioral issues are absolutely stemming from his back pain. She also recommended keeping his BOT sheet on at night, lots of long and low trot work this week, and treating for ulcers again. We also pulled blood for lyme. If nothing comes from this course of action, then she will refer me to the vet hospital for a bone scan. On a very positive note vet said that after we get his back muscles strengthened and comfortable, she sees no reason at all as to why he couldn't jump again, as his hooves look great.

I know I said that I was done, and I couldn't do it anymore. And I really couldn't. I am so fed up, but for some reason when the MSPCA said euthanasia, I just couldn't. Not after seeing him run around the paddock with his buddies. He's just not there yet, and neither am I.