Please wish me well. This is my fourth day of everyday visiting my Mother-in-Law in her 90s. My husband and kids are on well-deserved holidays, especially deserved by my OH who is the main carer. MIL is in a good physical condition at the moment. She has even started going out to the garden and cooking dinners.
As I wrote here https://www.carersuk.org/forum/support-and-advice/all-about-caring/caring-and-underlying-family-tensions-36147?hilit=tensions
MIL has a lot of negativity and from time to time she lashes out tonnes of criticism at me. Example: 'I have sacrificed everything for my son and now someone else benefits from that. I gave him money, time, washing, cooking cleaning and what have you given him?! Only children." 
However, most of the time she complains about other people, mainly about these evil girls that seduce and suck up all money from these poor helpless men.
MIL is indestructible. Yesterday, she went on and on with her stories (many of them really interesting I must admit) and complains for nearly 3 hours with short breaks for sips of tea and bites of a cake. At my 45 years of age I would not be able to move my jaw for such a long time!
Today, I am a bit stressed that she can be criticising me again. I know this is her problem, not my problem. Anyway, listetning to any kind of negativity for hours does not do good to anyone. I would rather spend less time with her than I do now (2-3 hours every day), but on the other hand it is worth it, so my OH had peaceful holiday. Otherwise, she is really capable to be on her own for most of the day which is really good.
Thank you for reading that.
Fourth day today - that means you have already managed three days (maybe even four by the time you read this) - well done! It was never going to be easy, but you have been heroic. Look forward to your real holiday later on.
Can you find something nice for yourself when you get home again? Something good to eat, or read, or listen to to reward yourself?
If she is OK on her own most of the time, can go out in the garden and cook, why on earth are you subjecting yourself to two or hours of this negativity every day?
What does she actually NEED you to do that she cannot do herself?
Would it not be possible to just phone and ask if there’s anything she needs instead?
You know, this is a complicated thing. She is very anxious and paranoid (not in medical sense) about being left alone for long, about thieves, murderers. She is also unable to go anywhere beyond her garden gate as she is unstable on her feet and still to weak after her fall last Summer. She has got used to everyday visits and I am not in a position to change it now as I am only replacing my OH. But you are right, she doesn’t need me for a long time. It is up to my husband to shorten visits.
It is impossible to make MIL happy, I stopped trying to do that long ago. Can you imagine, my OH visits her twice a day, 4 hours altogether and she keeps saying that “My son just drops in to see me from time to time.” 
Having said that, today was not a bad day. There were some stories about bad people and life disasters, but generally we focused on watching a singing competition on the Internet.
Thank you for all replies, they are very supportive. It is so good to know somebody is listening.
You deserve a medal Jolanta. However, paranoia and anxiety can go hand in hand with mood swings too. Always remember that whatever today throws at you…tomorrow is another day and I assume that the ills from yesterday may get forgotten. Distraction tactics are good, but energy draining for you, so make sure you always have something to look forward to at the end of the day. You got through it. The words can hurt. But that is all they are. Only words. Try and pull back and definitely bite your tongue.