Well I got to say, I'm kind of with you on this. I'm 26 and I'm amazed at how quickly time is going now that, as you said, days become more alike because you don't have the breaks in school or whatever to look forward to. I felt it a little stronger earlier this year. I had a hot girlfriend, good job, etc. but I would still be wondering, "Man, is this all there is? Why doesn't it feel right yet?"
What job do you have? Is it fulfilling? Because that's where you're going to be spending a shitload of time over the next 40 years or so.
I'm a journalist and a writer, so the job itself is never the same, not on any day. And when it does feel the slightest bit monotonous, something remarkable (or remarkably bad) happens that reminds me that everything can change, be taken away, in seconds.
I realized this year that really the only things you ahve to look forward to when you're older are your experiences and what you learn, or are learning. This year I read probably 15 books, started a bunch more, kept my own column up, became a MUCH better boxer, learned to hit the speedbag (something small that you will always be able to show off for the rest of your life), got much more into camping and deep sea fishing, shot a gun for the first time (ranging from a 9mm to an AK 47), and covered my first professional boxing match last week where I got to meet Roy Jones Jr., Max Kellerman, Jim Lampley, and a host of other very famous people.
None of these things really fell into my lap. I had to put in countless hours to learn to hit a speedbag, I had to go out of my way to learn to shoot, I had to connive and con my way into covering that fight... but without it, what the fuck would I have to look forward to?
I have two suggestions. One is to read about the life of Hemingway, and the other is go to this site: http://artofmanliness.com. Take one article a week off that, and really try to learn about the subject they talk about, or learn a skill that they outlined (on your own, or seek out a teacher). Something minor like learning about Baroque art, learning French (as you said), learning to cook, box, grapple, or start a fire without matches can be something that makes you feel like you've accomplished something, and gives you an itch to accomplish more.
As for Hemingway, regardless of what you think about his writing, the idea of living his type of life is what guys should strive for. When you look back at your life, would you rather have the pictures of a standard life, or one sthat have you posing next to marlin you caught or lions you shot, with your boxing gloves on, with a shotgun in hand, in the mud in Europe during a war and later in the capital of a Carribean county posing next to one of the most infamous dictators in the century?
Whateve you do, DO IT. Don't be complacent, don't say you'll learn to cook tomorrow (the worst word ever, in my opinion) or you'll call the martial arts/leatherworking/whatever school "when you have the time."
Just do things. Don't lament that no one will do them with you. Trust me, if what you're doing is cool enough, people will want to fuckin come.