Well, my motivation for learning any language (or any other thing for that matter) is simply fun. I normally do not think about the usefulness of something when I first start learning it. I absolutely love learning languages simply for the pleasure of learning them. So, before taking up anything, you need to analise why you would like to learn it, and that will be your main reason to keep learning it. Every time you get discouraged, remember what your reason is.
You need to be well organised when planning your study time. Do it every day, even some minutes, but be constant.
And finally, try to find materials you enjoy learning from. I don't know what your learning method is (if you use grammar books, or web sites, etc), but you must enjoy studying. Try to practise using topics you like, as animals, science, tv etc. For example, if you study grammar, you can try to find the new pattern you've learnt in an online magazine/article of the topic you like (google is quite useful!)
As for your questions:
1) No, I've never felt they are useless. I sometimes feel certain topics are complicated, but that's another reason for me to work them out!
2) By reading things I like, talking with people in that language, etc.
3) I'm self-taught. I've bought some grammar books for the languages I'm learning, and I usually chat in those languages on internet. I like reading, so I read a lot, and that's useful for improving grammar, vocabulary, etc.