Learning Korean is tough; there are no two ways about it. When I first came to Korea, I planned to conquer Korean in two years and then do the same with Chinese and Japanese after that. I figured that would make me something special. However, it didn't take me long to realize that Korean was the only language of the three in which I'd manage to achieve proficiency, and that improving those skills would be a lifelong project.
I've met other Westerners who have started along this journey but gotten discouraged. Here's a typical question and my answer to it.
Q: Will Korean skills boost my career opportunities in Korea?
A: I would like to say that the answer is a definite "yes"; however, as with many aspects of life in Korea, the answer is nuanced.
It can be a discouraging reality to accept, but your Korean abilities are not going to fast-track you in your career in Korea. In fact, though Korean skills may work in your favor if competing against someone for a job who doesn't know Korean, it is unlikely your Korean will be a crucial factor in the hiring process, no matter what job you apply for.
In general, if you're from an English-speaking country, you'll likely find your best job opportunities in positions that take advantage of your English abilities, and once that happens, your Korean is no longer an asset; in some cases, it's a disadvantage if your prospective employer is concerned you’ll be more focused on learning Korean than on helping your coworkers and/or students improve their English. Your employer is more likely to appreciate your Korean abilities if they didn't hire you for your English, but your Korean still won't be a key factor in the hiring process.
Way back when I taught English, I remember trying to inject Korean into my classes. Students (understandably) hated that. Later, when working in the LG Group as an editor/writer (and later, off-and-on translator), I was given strict instructions NOT to speak Korean in the office. I recall trying to refer an American friend to a position that had opened up in the company and when I mentioned to the director that my friend was fluent in Korean, he flat out told me nobody cared about that. Even now in my Ph.D. studies at Hanyang University, the semi-frequent job offers I get from the university to teach always involve helping the university fulfill the government-mandated requirement for classes taught in English and I often sense disappointment that I'm so focused on doing my coursework in Korean.
Having said that, I can think of some situations where your Korean skills could be helpful. The first would be where you have been hired for your English skills but where your Korean abilities let you understand and participate in office communications. This may make your more effective and fulfilled in your job. But as a foreigner, you won’t be on a career path to which you can apply this effectiveness and so the main benefit is likely to be found in helping you avoid some of the feelings of isolation that you'd encounter otherwise. But plenty of non-Koreans without Korean skills have managed their way through those situations, so it's not absolutely necessary.
You may also find that your Korean skills let you discover roles that wouldn't otherwise have existed. Your ability to leverage these roles would then be the determining factor in where you go from there. For example, being good at Korean can generate a lot of curiosity and if managed strategically may lead to hidden opportunities. I’ve encountered a few of those, such as being appointed Foreign-Investment Advisor to Gyeonggi Province when the Governor was impressed with my Korean. But networking opportunities are not the same as a career path. Besides, English skills are also a point of curiosity with Koreans and this can open doors, too. Thus, being stubborn in using Korean can close some of those English-oriented doors of opportunity, as well.
One more observation.... Even though speaking Korean is not going to make your career, the longer you spend in Korea without learning the language to a certain degree of proficiency, the more of a drag it may be on you, both personally and professionally. One reason is that Koreans may question your commitment to the country and your diligence if you never move beyond English interactions, and this can affect professional perceptions, too. Thus, speaking Korean may not help much, but not speaking Korean may also not be so great. Eventually, those who don’t learn Korean (and many who do!) end up “moving on” and not sticking around.
Speaking Korean often feels like a “brownie point” earner more than a killer resume skill. It’s a career asset if used strategically, but even that's not easy. And social pressure in Korea can provide a compelling excuse NOT to learn Korean.
I would say that if career opportunities are your primary motivation to learn Korean, then it’s not worth the trouble. The Korean learning process must have deeper value for you in personal ways -- such as the satisfaction you get from communicating in a difficult language and cultural context -- and that requires a special love for Korea.
When Koreans learn English, they can travel the world and meet people from many countries; when we learn Korean, we can... well, we can travel around Korea and meet Koreans. Ultimately, learning Korean is a niche endeavor that narrows (but deepens) your options.