I'm very sorry to hear about your Dad. And also sorry to hear about you. You seem like a very caring child, with a strong conscience, and I'm sure your parents think a lot of you and are comforted by what you are able to do.
There's nothing like hindsight. It makes us wiser, and it mucks us up. And if you're already feeling guilty it gives us a nice big stick to beat ourselves with. There's really no right or wrong amount of support you can give your parents, or right or wrong amount of sacrifice you have to bear. Or right or wrong amount of guilt you should feel. This of course makes those of us who feel we should sacrifice more carte blanche to feel even more guilty. My partner and I look after an 87-year-old, and I always say we can only do what we can do (but only sometimes do I believe myself).
Are you on your own? Do you have family responsibilities yourself (partner, children)? What would you sacrifice by being closer to your parents? - but more importantly, what would they gain? Do you have siblings, and if so are they taking their share or do you feel you have to carry the burden yourself?
Practical stuff. Talk to your local Family Carers person: they can help you practically and emotionally. I wonder if your Dad now could be/should be on meds of some sort? Plenty here will tell you that it takes some time to find the right ones. Does your Dad have a Parkinson's nurse? (more practical, more accessible than the neuro) How is your mother coping? Does she need help? I'm thinking that if te pair of them can get stable and comfortable on local resources, it'll mean your visits can be more supportive.
Does this help? Feel free to ignore it all. It's not advice - what do I know? - it's just some thoughts. But don't let that voice in your head beat you up, otherwise you'll be no good to anyone.
Take care. Let us know how you get on.