Primer can be useful to help paint fresh paint bond better, or there are also additives that can be added to help it 'stick'. What grade paper did you use to sand the existing, how much and what paints are you using, acrylics or enamels.
It also looks as though you are using normal 'white' masking tape??? Try some of the less harsh 'blue' or 'purple' tape that is designed for low tack masking. Make sure you get a decent brand.
Also how long did you leave between the black and doing the masking? Paint takes a while to 'harden' probably more than you would think, and fresh paint is more likely to peel.
Basically dont skimp on prep and esp in masking, it will catch you everytime..
Least you can fix it up and its always a great learnign curve painting your own car...

Shaun