Hello,
I am sorry you feel this way.
You say you are qualified and have good experience. So does the Mentor Nurse. Just because she qualified 400 years ago, doesn't mean she is a fossil. If she has kept up with all CPDs etc I am sure she is very capable of doing her job.
Yes, a Mentor Nurse should have trained for being just that. I say should as I don't know about your Mentor there.
Now, why don't you write down the instructions you are given. If you are given other instructions about the same matter, just read out what you have written down and ask if that is wrong or have been changed. (write down who told you as well...)
It is difficult sometimes and if you feel you don't get any help it is even worse. But, don't forget that you can ask about things as well.
It will show that you are interested and want to do things right.
A two way communication is the best, and if you are doing your best, they will notice and you will soon find that all is much better.
Good Luck!