Hi Shamir.
First of all, don't worry at all if you can't write as good as Mozart or Beethoven at the moment, and also don't worry if you think you would never be able to write as good as them. As Edvard Grieg once said, "Artists like Bach and Beethoven erected churches and temples on the heights. I only wanted... to build dwellings for men in which they might feel happy and at home.", now we all know that Grieg was a fantastic composer and he wrote so many masterpieces, yet he kinda 'confessed' himself that he was nowhere near as good as Bach or Beethoven, but this doesn't mean that he wasn't a brilliant composer. You must also consider the fact that Beethoven was studying composition since he was 9 or so, and Mozart was even younger when he started. Plus, Beethoven was almost 24 when he wrote the String Trio you mentioned.
Also, you are only 14 years old! Many of the great composers (including Berlioz, Schumann, Stravinsky, and Tchaikovsky) wrote their first complete pieces when they were 20 or even older (Stravinsky and Tchaikovsky were almost 23-24 when they wrote their first serious and long pieces) and many of them were even older when they wrote their serious pieces (Borodin was almost 30 when he completed many of his pieces and Bruckner was 38-39 when he wrote pieces like String Quartet in C minor and the study symphony).
Now let's talk about your problems with composition:
As far as I can understand from your post, it seems like you're trying to write melodies out of nothing, and it's one of the mistakes I did when I started composing music almost a year and a half ago, and I got a 'composer's block' after writing 4 short pieces using this method of writing melodies and then harmonizing them (the pieces were all about 2-3 minutes long, and they had many flaws although I had spent several weeks on each of them).
The best advice I can give you is to start studying harmony, because this is what every great composer did. You should understand how harmony works before trying to write melodies, it makes the process very easier. The book I recommend for starting is Walter Piston's Harmony textbook, it's really clear and simple and it has tons of exercises and examples from the common practice period. It also covers a lot of different topics from the very basic ones such as intervals and how chords are built, to the more advanced ones such as altered chords. Once you could understand what he says in that book, you can continue with Arnold Schoenberg's 'Structural Functions of Harmony', which is a book that I'm studying at the moment and it's definitely one of the best textbooks on music that I've ever seen. In this book he introduces the idea of regions and how you can use the chords of other regions of a tonal center systematically to create interesting progressions. He also explains a lot of great stuff about transformations of chords and vagrant harmonies and how you can use them in your progressions. Also, in the later pages of the book (which I haven't gotten to read yet) he illustrates a lot of examples to explain the concept of extended tonality and finally begins teaching some small forms (sentences, periods, etc.) and shows some examples from the repertoire.
Now, after you'd finished studying harmony, you still shouldn't expect yourself to write long pieces, because you should first gain a clear understanding of musical form. There is another book by Schoenberg, titled 'Fundamentals of Musical Composition', which I'm going to study myself after the book I'm currently studying. And just by reading very fast through some of the pages of this book, it seems that he's going to explain how someone can write a long and complete piece out of a basic motive. In this book, he starts by explaining how a motive is written, and then how you can write different variations on that motive, then he gives a detailed explanation of how basic themes (sentences and periods) are written and illustrates a lot of examples from the masterpieces of the repertoire. After all of this, he explains the small ternary form and also how you can develop a theme (where he writes 25 different developments on an 8-measure theme). He also talks about other things such as the sonata form and many other things. I think it's a book which is going to help myself a lot in the future.
My final advice is, just keep studying and practicing composition and never give up. It might even take you some years to figure everything out, but I think it's definitely worth it.
Good luck!
P.S. You can also start by studying Schoenberg's 'Theory of Harmony' instead of Walter Piston's book, but I don't recommend it.