This is the saga that every momma faces! PB & J can be good if you doctor it!
First off no regular jelly. Get the simply fruit that ONLY has fruit in it
Secondly start off with something aside from peanut butter. Say oranges, apples etc. They give her the sandwich. This will encourage her to try new things AND reward her for doing so.
I finally got my 3 year old out of wanting butter (what he calls peanut butter) and jelly sandwiches by serving other things with it and eventually he found the next thing that he's going to fixate on for a bit.
All kids have their one favorite as do adults. I would eat potstickers every day of my life if I could. Kid's just need to learn a little self control when it comes down to it! (heck so do I!)
And it could be worse. She could be asking for cake every meal!