how about instead of just refusing say "I don't wear chains because they make me uncomfortable but I would love a ..... ring, key ring whatever. That way she sees you love the idea that she wants to buy you something to wear and it is something that you will wear instead of sitting on a dresser or stand.

There are many reasons why people don't wear specifc types of jewelry be it allergic to certain metals that they are made from, or the type of job they have or sweat build up along the jewelry line of wear. so you don't have to go into detail that wearing a chain would trigger flashbacks and panic for you.