It is so frustrating to put in work on a project, then realize it's not working. But it happens to all of us.A general rule is if the beads look good together just out on your worktable, you'll probably also like them in your piece. Try to have an accurate representation of the numbers of beads. For example, if your work will have equal amounts of two colors, don't group 20 beads of 1 color and just 1 of the other. You may find that what makes a nice accent won't work in larger quantities. If there will be a lot of wire, or a ribbon or chain, have some of that on the table as well. For a necklace, I sometimes lay out all the beads in order - sort of an unassembled necklace.
Pay attention to the colors you like together in other things, whether the natural landscape, paintings, clothing or anything else.
Most important, remember that any craft is worthwhile as a process - not everything you plan will work out. But I hope you will enjoy the work, and your design instincts will get stronger with use.
Best of luck to you!