I assume you have a multi-tool for different sizes. So you need the one that just fits inside the fitting. Put the tool in, then push the fuel fitting IN, the pull it out. Wiggle and repeat until it comes out.
Once it's ready to come out, it will just come right off, no fuss. Until then, you haven't released the spring yet.
Scott
I had the same issue with my first go at the fuel rail and lines. I have the white plastic pieces, I kept putting them on the rubber part of the lines and couldn't figure out why it wasn't working They go on the metal part (the fuel rail side of the fitting) and you push the tool into the fitting to release the line. It's easy once you see it the first time.