I've often heard people say about their exes (or even their current significant others), "They just need to grow up." While you probably know your ex significant other fairly well, and it's quite possible you are entirely right, you (or anyone else) telling them, "You need to grow up," is not going to make them grow up. It's not going to change the fact that they are immature, and it's not going to help them get their lives together. If anything, it only pissed them off that you claimed they are immature, especially if they know you're right. If they know you're right, it will probably just make them more mad at you for pointing out their insecurities.
As for the growing up... if only it were that easy. Some boys (or girls) never grow up into men (or women). They kind of just reach a point, and that's it, they just kind of reach a plateau. But you shouldn't have to lower your standards or have to ask for less. And most defiantly not sit around and wait for them. While you might know what you want out of a relationship, now is the time to figure out exactly what you DON'T want. Make a list of the all the things you DON'T want in your next relationship. Things that are major no-nos, and (this time around), you are NOT willing to compromise. For example, your ex hated dogs and you loved dogs? Your list should include, "Must like dogs."
Distract yourself as much as possible from the pain - now is the time to work on yourself and your personal growth, figure out the things that make you happy and do them as much as possible. Usually, during the day, you have enough to distract you from the pain. Whether it be work, the gym, or friends. At night is usually the worst - especially if you were living with your ex, and are now on your own. Part of it due to the fact that you have a lot of thoughts going through your head. Part of it due to the uncomfortableness and awkwardness of sleeping by yourself. While some people rely on sleeping pills, I have heard from friends of mine, that while sleeping pills work really well most of the time, after a break up, they are pretty useless. Not to mention, you still feel exhausted when you wake up. Part of that exhaustion IS emotional. The emotions are draining on you. Stock up on Sleepytime tea and Lavender oil, or something similar to help you sleep. The tea won't give you the hungover effect the pills would.
Think of it like a giant jigsaw puzzle. Sometimes you fit a piece that kind of fits, for the most part. But eventually no matter how much you try, it just doesn't fit. There IS a piece that fits, you just haven't found it yet.