I can read/write foreign languages fairly easily, but listening/comprehending (especially) is very difficult. Why do I suck at this?

Maybe because you have learn the language at school, but in your school they just always provide reading materials for you and ask you to write essays, but they seldom have activities that can improve your listening and speaking skills in a foreign language, such as talking to exchange students that speak that language, or watching movies in that language. And you can improve by doing activities like travelling, watch foreign movies that have subtitles in a language you are familiar, and when you watch movies/dramas, you can't just look at the subtitles, you need to listen too at the same time.

Becasue writing is SLOWER than speaking. Your brain doesn;t work fast enough

Because It might not be worth pursuing.