One of the most frustrating subjects is without a doubt mathematics. You can work for years on a problem just to find out you're trying to prove the impossible. However, you just have to remember that what you're doing is probably going to benefit in the long run - if nothing, it's giving you a lot of experience. I get a whole lot of boring problems as an undergraduate that require a lot of tedious and long-winded proofs/calculations, but that doesn't mean that they're pointless. I just get through it by looking into the future and being able to work on what I want to work on.