I’m so frustrated! My husband has been declining from his stroke. I put him back into therapy. He fell down on vacation. Any advice?

Please take a deep breath and realize that your husband more than likely will never get any better. The rule of thumb with anyone that's had a stroke is that whatever they're going to regain will happen within the first 2 years.
It's now going on 4 years, so unfortunately that window has now closed, and you must accept that, as getting resentful and anxious will only make things worse for you both.
You must now learn how to just make the very best out of whatever your husband can still do and treat him with love and respect, the same way you'd want to be treated if the tables were reversed.
And if his care is getting to be too much for you then you may have to look into placing him in the appropriate facility.
My husband had a massive stroke back in 1996 at the age of 48, just a year and a half after we were married. He lost his ability to walk, talk, read, write and use his right arm/hand. With PT, OT and speech therapy he did learn how to walk with the help of a brace on his leg, and eventually could say a few words and very short sentences, but never learned how to read or write and never could use his right arm/hand.
Was it hard for me early on? Of course, but we both just learned to make the best of it, and to accept our "new normal." And that's what you must do, as life is just too short.
Perhaps you may have to get him a transport chair for when you're out and about so you don't have to worry about him falling. I used one of those with my husband for many years when my husband became a fall risk.
And you also may just have to let him sit in his chair and watch TV, if that's what he prefers, if that will keep the peace in your home.
My husband in his later years didn't want to leave the house as it was just too much for him and he was just content to sit in his chair and watch TV all day. And I had to be ok with that. Now that doesn't mean that I didn't go and do with my girlfriends, because I am a people person and like being out and about.
So make sure that you're taking time away to do things that you enjoy as that will certainly cut down on your stress levels and give you more patience to care for your husband.
My husband died in 2020 at the age 72, of vascular dementia, and I miss him every day, so please just enjoy whatever time you have left with him.
You might want to see if there is a stroke support group in your area as that would be helpful as well.
I wish you and your husband the very best as you both get used to your "new normal."

Honestly, I’m surprised insurance is paying for therapy anymore. Usually, without documented progress it stops. And yes, sadly, after this much time his stroke recovery is complete. What my family didn’t realize when my mother had a series of strokes is that they often leave depression in the aftermath, and that depression needs treatment. Is your husband being treated for this? If he is and is still depressed, then the meds need to be reevaluated and probably changed. Look into post stroke support groups in your area, for both you and your husband. They may be able to provide information and resources to help. Being angry and resentful is only affecting you, so seek time away from your husband, even if it’s simply a walk outside daily or time talking with a friend. You matter too.

So sorry for your situation. It's very hard to get our loved ones to do PT for us. My mom was impossible so I have the caregivers do it.

I am no expert on strokes but it kind of seems like 4 years is a pretty long time and that he probably isn't going to get any better, in general. Is the PT helping? I would make him keep going as long as the insurance continues to pay for it.

There is usually some kind of home health care agency around. They might be able to send you someone to help with PT, etc. There is often a minimum hours, like 3 or 4 to make it worth the caregiver coming to you.