If you can handle something, it's manageable. An overworked college student might decide to drop a horrible chemistry class to make the semester more manageable.
Anything that you can accomplish can be described with the adjective manageable. It's also used in a slightly different way, to talk about something that can be controlled, like a tamed lion or a smooth and tidy head of hair. The earliest use of the root word "manage" related specifically to controlling a horse, influenced by the French word for "horsemanship," manège.