Just take it slow and easy and be gentle. Pry against the old pads. After you get one side out, block the space with a new pad and pry/push the other piston back. Take your time and get the pistons pushed all the way back and then the new full width pads will slip right in. Sounds like you are having the same problems I had with my first brake job. It was "pry and try" (fitting new full width pad), "pry and try", over and over again, until I finally figured out I didn't have the pistons pushed all the way back. I do have one of those generic tools by Lisle but it doesn't fit well on this particular caliper. Watch your fluid level in the reservoir. Cleaning up spilled brake fluid is a real pain. No need to fiddle with the bleeder. This is only if you decide to flush brake fluid.

Sounds like your first brake job. You might have an experienced friend watch over your shoulder.

glenmore
1991 300CE
2000 C280
1990 LS400