Service Dog Situation Update

A friend offered to watch Chakotay so I could "test drive" some foster dogs from the local shelters or rescues and such. I have been looking for months, much how I did when I went looking for and found Journey.  Nothing in the local area young enough, big enough or heartworm negative. I came across a listing for a dog claiming he was a Lab X Mastiff mix, which I thought was perfect for size and possible good characteristics. Went to see him... the dog was smaller than Ember, and more of a Pitty mix.  He was beautiful and beautiful personality, but too small.  So I asked to see what else they had.  Only one really fitting big enough was an American Bulldog. Had good personality and some basic obedience, we tried cat testing in the cat room and he didn't seem interested in them and even went up to sniff one cage and got nailed and didn't even react, just turned away.  Perfect I thought. Went back to test him with Ember and they liked each other. Okay, willing to give him a chance.  He doesn't weigh 70lbs like we thought by the size of him, but 103lbs!  Mega perfect!  Find out he's heartworm positive. Nrgh. Okay. Shelter is willing to work with me, so I agree to try him on foster.  Always cautious when introducing strange dogs to my cats, especially one as big and strong as him....he gets excited when he realizes they are there, and it is prey drive excitement.  I tried a few different approaches to get him to calm and ignore them, which he actually caught on to amazingly well when realizing he got yummy things for ignoring the cats.  But he kept doing the eerie freeze/silence/stare predatory stalking behavior thing.  He also tried to nip at Wasabi through the kennel  when Wa came over to politely say hello ( if the bars had not been there between them the dog would have made contact, which is very dangerous ). I can't risk that with my cats. I can't risk he go after some other random animal in public and risk dragging me down or worse.  I am totally fine working with large, powerful dogs. I have all my life.  But at the same time I can't risk injury to myself to fight against trying to reshape that kind of prey drive on a dog that big.  I hate to say it, but I don't have the time or energy or finances to do so.  So I will be bringing him back tomorrow.  Back to square one. It is getting harder and harder for Ember to do what I need her to do for me, and I am afraid she'd work herself to death if I didn't stop her.  I am afraid of facing being in public without a service dog after all the freedom and peace of mind I have had with my partners, but I am running out of time and options. Fundraisers don't work ( I can't even help save a cat with a broken leg fast enough).  The waiting list for CPL or any program is going to be longer than I can wait, and I don't have $5k-$7k just to travel for team training.  I don't even have $2k to get a well bred pup with health guarantee (if something is wrong breeder will take it back and replace it and their parents are screened for HD/ED and temperament tested CGC and above).  I feel like just totally giving up. I can't do it anymore.