Rescue hiding under bed, scared to go on walks, starts shaking when he sees people

So about a year ago, my roommate and I adopted an 8-year-old Spaniel/Terrier mix. He had been rescued from a hoarding situation about 3 months prior to us adopting him. The shelter representative said he would probably never be like a "normal" dog. Anyway, he's been with us for about a year now, and while he has improved in many ways, it feels like we've hit a wall. I take the most care of him, with my roommate occasionally taking him outside for a bathroom break. He starts shaking when he sees new people, but about 6 months ago, he'd gotten used to my roommate and wouldn't shake when we all spent time together. Now, whenever she's in the same room, he'll start shaking.

Additionally, we used to go on walks every day, and he used to be so excited to go. It was so hard to get him to come back inside, that's how much he liked walking. Now, we'll make it maybe about 50 feet down the sidewalk, and he'll panic and sprint back to the apartment.

Another thing that I have a concern with is that he hides underneath the bed. All day. He'll occasionally come out if we're cooking food or to get water, but I have to drag him out from underneath the bed whenever I have to take him outside to use the bathroom or to feed him sometimes. After this, he'll spend time with me on my bed, but if I leave my room, he won't come with me. He'll hide underneath the bed again as soon as I leave. He'll spend time with me in the living room, but if I'm in the kitchen, he'll scratch at my bedroom door to get in. Sometimes, he'll start digging at the carpeting when he's really stressed.

It's been almost a year, and we were improving a lot, but I feel as if he'll never stop hiding underneath the bed, and we won't be able to go on walks together again. Does anybody have any recommendations?

I appreciate all your help!