I recognise these feelings so well - you can tell yourself till you're blue in the face, and others will also tell you, that it's precisely because you're so dependable that people take you for granted, but it's much harder to change the situation than it is to acknowledge it. I have been interested to note that both Jesus and the Buddha recognised the importance of loving yourself. as well as 'your neighbour' or 'every sentient being' as they put it respectively, so it's clear that this dilemma must have presented itself to people even 2,500 years ago. It is all too easy to think that we are only really helping others if we forget ourselves, which we've often brought up to see as selfish. But I have found remembering what these two great spiritual teachers said has helped me take action and to ask for help. I had radiotherapy and chemotherapy nearly 44 years ago, and though my cancer is now very old news, the resulting damage to my body from the treatment, which couldn't have been predicted, is not. I can now walk only with difficulty, can't hold my head up straight, and have nerve damage which means I can't feel my feet and often lose my balance and fall. I''m 67, but am less capable around the house than my husband's mother, who will be 100 in two weeks' time. So I have had no alternative but to cut back on some of my 'stalwartness', chip away at my sense that my identity depends on 'being there for other people whenever they need me' and ask for help for myself. I feel ashamed when I don't offer to move the chairs after a meeting, or to wash up after a U3A group meeting at my house, or even when I have to ask someone else to bring their own tea through to the sitting room - I feel ashamed even though I know that I would probably fall over and break the china or spill hot tea into a friend's lap. But I've had to learn not to let that habitual shame stop me letting people know that I need help, and more importantly, I've discovered how happy other are to help me in their turn. Just as I like to be appreciated, so do they.
All this is a very long-winded way of saying that your feelings are very understandable, and probably common to many of us, but that this doesn't mean that you can't take the matter into your own hands by reframing the situation, even though it requires quite a considerable amount of emotional effort and faith in your fellow human beings.
And as someone with diabetes (Type 2) myself, I would feel more than a little grumpy if the demands of others meant that I wasn't able to take the action I needed to control my own health. Your family needs to know that if you can't get to the gym as often as you have found helpful, you will be at greater risk of going blind, losing your feet, and having to go on dialysis. They wouldn't want that on their consciences, would they? They would be far more likely to say "Why didn't you tell us?" than to call you selfish. Tell them - I'm right behind you! Good luck, and make sure you let make everyone muck in at Christmas. Have a happy one!