Kelly, there are 2 ways that I know of to do this and you are half way there on one of them. Redo all your measuring but get it exactly accurate--to the 16th. Measure each shelf to match their respective measurements. As long as the other ends of the shelf are at 90 degrees, the angle should come out pretty darned close. To get really accurate, you can use measuring sticks instead of a tape measure. Cut 2 square pieces about 2 feet long. Put them next to each other with the ends exactly on the length you need to measure. Make a pencil mark across the pieces. Keep the pieces and the pencil mark aligned and the ends will always be the same distance apart. You will have to cut one of the pieces in half to do the 19-1/2" measurement so do it last.
The other way is to take a bevel gage and find the angle of the walls meeting. Check the angle on a protractor and take half of it. Use the protractor and bevel gage to mark half the included angle on each board and saw cut to this mark. Then measure for the length of each board. Cut square to this length and you should have it.
You can take cardboard and make prototype parts first, if you like, and check the fit. When you get the cardboard pieces right, use these as templates to cut the boards.