I cannot see you tipping backwards at 15% unless you have shifted your weight by standing and shifting back. I know it is not a ton of help to keep hearing the same thing LOL,but stay seated, keep that relaxed feel on your handbars (use the flat part) do not pull on the bars but focus on using your power to turn the pedals. You are coordinated enough to get out unclipped, do you think you really could have managed that on one wheel? Try less steep longer inclines to get some practice if your basic problem is that you need more hill riding. I think you must be panicking and letting your head take over the ride. Can you find longer inclines for hill work? 15% is less daunting when it is just part of a long climb, when it is without build up I am sure it can appear scarier.