You get the point.
We're heavily influenced by fiction: stories, books and movies that feature a strife and end with a presumption of 'happily ever after'. This is not at all how one should look at life. Sure, accomplishment is great, but if you're accomplishing something and expect change to come naturally after the achievement is completed; you're suffering from a common misconception.
"I just need to get this job to be happy. "I just need to get this degree to be happy." I just need to get this girl and i'll be happy." "I just need to lose this weight and i'll be happy."
or even:
"If i would win the lottery, i'd be happy."
It looks to me like you've either achieved a goal and seen nothing good come of it, failed to achieve some dream that you imagine would fix all your troubles or most likely: stopped for a while and realized the road doesn't really lead any place worth caring about and that nor would any other road.
What you've seen isn't the futility of life it's the futility of the system we're taught from our earliest days. "Just hang in there, you're almost done!" We've built a society based on pursuit and postponement of happiness - always something to look forward to so you can put up with the now.
Just forget about that! Do what makes you happy. I think you've simply not had icecream for dinner. You probably haven't 'cause "that would be dumb." Well, i'd argue that if even the slightest part of you thought "that would be fun", hushing that voice down is a crime! You should listen to that voice, but in moderation. Having icecream for dinner every day would be dangerous, so would doing heavy drugs so don't listen to every whim the voice may utter. But if it's not too dangerous and your only reason for not doing something is "that would be dumb", do it!
Take quitting for instance. I'll share my experiences. I was always told quitting is bad, that if you quit something you'll just have to start over and once you start quitting you'll never stop. When i ended up doing something exhausting once, i decided "fuck it, why not?" and quit. As much as i enjoyed the freedom of doing nothing for a while, i realized i enjoyed the feeling of accomplishment more so i started doing shit (successfully) again and instead of not quitting simply for the reason that 'quitting is bad, mkay?' i appreciated what i previously resented. Much like one would enjoy mashed potatoes after a binging of vanilla icecream.
Appreciate the little things, if you don't - try substituting them. Society isn't as bad as it seems, it's the way it is for reasons (mostly). You can even substitute society but once you have kids the securities and comforts of society become quite apparent.
Just do stuff you like. ^^ Or stuff you think you'd like. It doesn't have to be meaningful, your own happiness is what's most important.