Try to find a language that you can stay motivated to learn. Is there a language you maybe have more reason to learn, like maybe it's spoken in a country where you visit a lot, or would like to visit?
I don't really have this problem, I never really get bored of languages. Except in a few cases where I have learned a bit and then got bored and given up, that was when I didn't really have enough motivation. For example, the language I progressed least in was Finnish. My initial interest was because I had about two Finnish songs on my iPod that I liked, which it turns out isn't enough for me to stay interested. I probably wouldn't ever find myself in a situation where Finnish would be useful to me (though I probably would visit Finland at least once in my life, the same could be said of most countries, because I am fascinated by foreign countries in general). My main problem is not so much that I get bored of one language, but that I get interested in new ones, so I attempt to learn loads of different languages at once, which means I don't really do well in any of them. I don't do that anymore, though, I'm focusing on one at a time, it's much better this way.
My main language is Spanish, which I chose because I have visited Spain a few times and would like to visit again, and because I love Spanish music, and because it is one of the most widely-spoken languages on the planet (I think it is fourth, after Mandarin Chinese, Hindi, and English).
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'El reloj, no avanza
y yo quiero ir a verte,
La clase, no acaba
y es como un semestre"