Concur re “long, hard slog,” that should be what you prepare yourself for. As to literary agents being “idiots”? As with all professions, there are people who are good at their jobs and people who are not. Obviously, a writer wants to look for reps who are good at what they do. They also should find reps who they feel ‘get’ who they are as writers and the material they write. That doesn’t necessarily mean they have to be nice. In some ways, you want agents (especially) to have some bulldog in them in order to negotiate the best deals possible for you.
The most solid advice I’ve heard about working in Hollywood — and this extends to everyone involved in a writer’s life including talent, producers, execs, reps — when you find the good ones, foster those relationships and continue to work with them.