I'm so sorry you had to put down your 20 year old - but she lived a full life!
Yup, sometimes it's just their personality. But I'd take Wendy's advice and run with it. If you free feed dry food, I'd start giving them either "breakfast" or "dinner" - whichever is most convenient for you - but make it a meal of wet food. Stay with them while they eat - but sit on the floor with your side to her. And when you reach out to pet, do it slowly and palm down.
I'd still get a t-shirt really good and sweaty and put it under the dry food dish.
You can also consider purchasing Feliway. It's a synthetic hormone that mimics the "friendly" markers in cats' cheeks. It may help calm a scared kitty.
You can also consider purchasing a Bach's Flower Remedy - either Calm & Serene or Rescue Remedy. Add a few drops to the water dish after you clean it every day.
Both are available here: http://www.catfaeries.com
though I'm sure Feliway can be purchased cheaper other places, and sometimes you can find the Bach's Flower Essences at health food stores.
Also, baby food (all meat - we use Gerber's chicken) is a great treat. Most cats can't resist it. At first, put it on a spoon and hold it out. Try petting while she's eating the baby food.
But basically spending as much time in the same room she is - down at her level, sitting on the floor and doing stuff - working on a laptop, reading, knitting, folding laundry - whatever you do or whatever you can do - and letting her just check you out will help too. Sit with your side to her and just ignore her! If she comes up to check you out, just keep ignoring her, and limit the pets to treat or dinner time.
She'll come around!
We have one kitty we rescued that took a year before we could pet him. Then it was just one stroke on the cheek. Six years later, he's a total love bug.
It's just that each cat is different, and there is no schedule for any of them.
But they are contrarians. Even now, the more attention we give them, the less they want. When we start ignoring they're around, they all want love.