I have flaps on my litter boxes I trained them when they were kittens , I
only had to force them through a few times before they learned how it
worked. It scared them a little when it hit their tails, till they mastered
how to control them. Missy is older best of luck to you.
Jan

I have never used a cat flap because my cats are indoor-only.
However...could you first tie the flap up until Missy gets used to it, then
lower it part-way, and then finally place it all the way down?  Of course,
the open flap will also be an invitation to outdoors critters to enter (so
are flaps of all kinds because other animals often do find the way inside),
so you will need to be cautious while the flap is fastened in the "up"
(open) position.

MaryL

[snip]

*Don't* force her through. You'll just end up with an annoyed cat,
possibly annoyed enough she won't want to have anything to do with the
endeavor.

Use *positive reinforcement* -- food, treats, affection.

We taught all four of ours to use the pet door by this method or some
variation thereof (some of our kitties watched the others and figured it
out faster):

Tape it open for a while and show Missy the opening. She'll probably be
curious (you know how cats are!) and quite possibly will go through on
her own. If not, take her favorite treat, toy, or bit of food and coax
her through. Repeat everal times. Reward her when she goes through. We
have four cats, so we left ours taped open for an entire day. With just
one cat, you could probably do this in an afternoon.

Then, close the flap. Coax her through with the treat/toy/food. You might
have to hold the flap up the first time or two so she sees that it opens,
and is not a wall. Toss treats or toys through if you're alone, or just
alternate coaxing her through the door with your partner. Repeat repeat
repeat! She'll figure it out eventually.

Note: If there is one, you might also want to remove the metal
strip/magnet/weight at the bottom of the flap -- it will still close
properly, and won't close quite as fast or hard -- a couple of our
kitties were a lot less spooked with the door once we did that.

Cheers,

Put her head through, then kick her up the arse.