Ashlee has been a 40min sleeper for as long as I can remember! It's hard because it doesn't give you enough time to do what has to be done around the house with 2 hands, AND have a break!
In a way, it's good because I know if Ashlee sleeps longer than that, then something could possibly be wrong.
As Isla gets older, you might find she grows out of that a bit (as Ashlee has - now it's wake up after 40mins, have a talk to her mobile, then drift back off to sleep for another 1/2 hr-40 mins again).
And don't doubt yourself! You are doing a fab job

don't feel guilty either, she's just communicating with you in the only way she knows how. You are doing your best to settle her when she's upset so that IS the right thing to do.