What is the best course of action?

Hey Vets and Vet Meds!

I'm in a moral dilemma. I have a dog (6YO, chihuahua mix) who has incredibly high needs. He is perpetually anxious around our other animals (and just, in general I guess), and has attacked them in the past. On the Dunbar bite scale, I would say a 1 or 2, if he manages to land a bite. He has also attacked me and my partner, however these outbursts he has towards us and the animals has decreased. I'm not sure how he got like this. Honestly, I'm newer to the family than he is, which is why I'm coming here first, rather than bringing this up to my partner first. I don't want to give her any stress about it, and I really don't want her to have to choose her dog or me, it's not fair to her, the dog, or me.

He was owned by my girlfriends father, who sadly passed 4 years ago. Our dog was latched at the hip with him. I was told that after he passed, our dog would look for him all the time, especially on walks, and especially in areas where he was taken in the past. This likely lead to him being as protective over us as he is, but it's just too much sometimes. Plus, he was trained (incorrectly) as a guard dog, and will try to attack one of us if we wrestle around with each other. Or if we simply reach towards him sometimes. Or if he's sleeping on the bed with one of us, and the other comes to bed (mostly me). We decided to take him after my girlfriend and I decided that her stepmom simply couldn't care after 2 dogs. He was flea infested, had an ear infection, and always a bit aggressive.

I just don't know what do with him. We can't afford to put him on antidepressants, and I know if we rehomed him, he'd be euthanized. I love the little guy, but I'm just lost. He's made progress with his behavior, but I'm terrified of investing in training lessons, just for it not to work.

Anyway, I apologize if this doesn't make sense. I'm working and doing this between free time. I'll answer whatever questions people have.