First of all, if you really hate a language, you should probably find something better to do with your time. I think learning a language solely for work or school, out of a sense of duty, should be discouraged. This kind of attitude poisons the way adults think about language acquisition, and when a person finally does have interest in a language, they might be turned off by past experiences.
That being said, we all know that sometimes knowledge of a language can make certain jobs more accessible. In most parts of the world, a command of English can mean a world of difference in terms of job prospects. While I don’t think the Anglophone world has this issue to the same extent, perhaps you have logical reasons to learn a language, but your heart is not totally in it.
Well, I had a similar experience with Spanish. Like a lot of American language learners (that I’ve noticed) I was opposed to learning Spanish for some reason (I believe that non-Latino Americans tend to not be interested in Spanish because the language doesn’t hold the same “status” as other European languages like French or Italian. Oh, the romance of those two! But Spanish? That’s not special or exotic. I think this is quite a problem, which I will discuss another time). Of course, Spanish was logical for me to learn: (1) I had studied it at the high school level, so I had a head start on some vocabulary, (2) I had already learned French, so I had double the head start on vocabulary and grammar, and (3) unlike French or the myriad other languages in which I’m interested, Spanish is useful right here and right now, in my own country. But, I started and stopped a few times while bouncing between other languages. I couldn’t stick with it, because I just wasn’t “into” it.
So, how did I fix this and learn Spanish to a high level? I barreled through, toughed it out, and, eventually, came to love the language for itself. That’s not a very glamourous answer, but bear with me. At the early stages of learning Spanish, I had incredibly convenient resources, and it was easy to go into “autopilot” for a bit. I used sample sentences from the “Word of the Day” at http://www.spanishdict.com/. I just copied and pasted into my SRS for a couple of months, pretty much only doing that and a little bit of listening. My brain was turned off so no reticence could leak through and make me rethink my decision to learn Spanish.
And what happened? Little by little, I grew attached to this language, and I began to like it. This isn’t the shallow kind of affection I referred to supra, “Ooh, this language is exotic, and people will think more of me if I know it.” No. I enjoyed listening to the language, I enjoyed reading the language. Everytime I came into contact with Spanish and understood a bit more, I felt a sense of accomplishment and ownership, and that feeling is fuel enough to keep me going further and further into Spanish.
Motivation wanes because people insist on viewing their target languages as something that belongs to someone else. It doesn’t– it’s yours! Make it a part of your life as much as your native language!
If you are studying a language you don’t like, or a language you’re burned-out on, shift your mindset. Set up your study techniques so that those are automatic– find an easy resource for sentences, commit yourself to one chapter of Harry Potter a day (this was key to my Spanish), or whatever else you need. And on top of that, cultivate an attachment to the culture, and feel like the language is yours. You have a right to this language, you’re spending the time learning it, and it’s as much yours as is your native language (although with tortured sentences like that, who knows what my native language is?).