I'm going to be brutally honest with you. You're not going to like it and you're not going to believe me (you'd protest until your fingers cramp), so it doesn't really matter, but I'll put it out there anyway.
You are not as mature as you think you are (Right now, you're saying to yourself: "But you don't know me, Daisy.") Overwhelmingly from what I've seen you post, you seem right about your age (And now you're saying: "But I just fool around on here. I'm not like this in real life.").
Yes, you believe yourself to be more mature than others your age. Guess what? The majority of people your age believe the same thing. I believed it of myself when I was your age. Then, at 23 or so when I had graduated college and had my first job, I thought: "Wow, how stupid I was then! NOW I'm really mature. *smugface*" By the time I was 30, I realized that I had still been a relatively immature idiot at 23-24.
And the thing is, I'm not alone. You'll hear this same story from many, many people over 30. (I know, I know... you're different than the rest of us... you really
are more mature. Just wait 5 years... and then another 5.)
Further, you shouldn't be surprised that a 24-year-old who dated you turned out to be immature - and yes, she does seem immature for her age. In general, a girl that age would be immature if she's attracted to a 19-year-old.
Like I said, I'm sure you'll argue... but I won't.
In all seriousness, my advice is to stop worrying about how mature you are, stop worrying about having a 'relationship' and have fun. Meet people - both female and male - make friends, learn about the world, life and all that.
Enjoy!