I read your question and was very delighted to know about your concern to abiding by religious rules, especially issues connected to your parents.
As u know in Islam pleasing parents is equal to pleasing Allah.
In regards to ur issue, you are aware that your parents' behaviour is due to their old age and sickness and no bad intentions are involved.
To solve this problem I came up with a few solutions:
1. Try and be more kind to them and fulfill their expectations as much as possible - this will lessen their complains.
2. Defend yourselves infront of those people who are confronting you by reminding them that they are your parents and nobody is concerned about them more than you.
3. Ask a couple of your friends whom you trust to speak to your parents after every few days and remind them about your positive qualities and the things you are doing for them. Maybe this will help in removing the negative and unpleasant thoughts from their minds.
4. If none of these work, dont lose your hope in Allah swt. Those things that are done for Allah swt will never go waste and unanswered. If He is pleased with you, others pleasure doesnt really matter. It is He who gives us izzat (honour/respect), therefore if ur obeying Him then He ll never belittle you infront of people.