I have become so bitter

It's really hard OP. I think a lot of people are in or have been in exactly your situation. It feels like a trap for you because the only end to it all is a sad one and you don't know how long the running around/feeling of responsibility and sadness will last.

It really makes you think about the health choices you make - which I suppose is an upside.

Echo what others are saying, I feel exactly the same. What ever I suggest such as light exercise classes in our village aimed at pensioners is met with negativity. They are bitter about nhs wait times, yet can afford private etc it is draining especially when you have your own life/ family too.

A few things have helped me, limiting my visits. This was difficult as we live in the next road but weekend tea / coffee visits have worked well.
Also, divide and conquer. My sister supports with financial things and I book medical appointments. Parents have found a taxi service they really like so that's taken the worry out of driving, and my husband/ BIL will take turns driving their car to work to prevent my dad feeling the need to give it a run out.

Im frustrated because I look at how different life used to be and I'm struggling with this reverse of roles.

Your anger and frustration is entirely understandable. I would find it very hard to bite my tongue, ageing can be challenging enough without self inflicted ailments. Agree with PPs that you should take a step back.

There's a lot of us out there who feel exactly as you do, OP. I agree with PP who says only do what is manageable for you to do, and get in touch with outside agencies. It's made worse, too, if they were shit parents, as mine were.

YADNBU for feeling angry and indifferent. Being in this squeezed situation of constantly caring is so bloody stressful and exhausting it feels like it can take years off the people having to do it.

You shouldn’t have to bear the consequences of other people’s terrible choices. It’s not your responsibility to derail your own life and run around wrecking your own health, happiness and peace of mind to look after people who’ve merrily fucked themselves up over an entire lifetime with no care for how it might affect those closest to them. You reap as you sow.

I know this sounds really harsh and horrible, but it’s borne of experience. It’s very different offering a high level of care and support to someone who’s ill through no fault of their own, and doing the same for someone who’s inevitably ended up where they are through their own actions.

Obviously it’s difficult if it’s your mum, but as @AtlasPine said, you don’t have to take it all on yourself. Maybe think about what would be a manageable level of help for you to offer, and look at other options to support her. Same for your FIL. Do you have other family members who can help out?

You know you don’t have to run after them. Refer to adult social services.

Having said that, I have huge empathy with you because that is far easier said than done. But keep intervention to a minimum.