@Bleu
I think you got a great start going here! Getting used to the tablet was, for me, the hardest part, you'll get there!
The best advice I have ever received in regards to art, outside of the tried and true "practice makes perfect," is to use references. Referencing some characters from the show (or comics, or even other OC's) can help you get a feel for the show's proportions, which you can later modify to suite your own aesthetic (once you've identified that certain something that screams pony).
Admittedly, some artists seem to think referencing is some sort of blasphemy or that using such somehow voids their style. However, I feel it is quite the opposite: references can help eventually develop a style by allowing you to take what you like or you feel necessary and cut what you don't. Working from memory is often not very efficient (especially if you don't have photographic memory) and can lead to various inaccuracies. Besides, nobody works in a vacuum.
And if you are already using references, awesome! You are well on your way then! If your at this stage, I recommend trying to "see the forest for the trees." That is to say, if one were to draw a heart, one should erase or otherwise block the symbol of the heart from the mind, and attempt to draw what is seen.
(also thanks for your kind words on my piece! Don't sell yourself short, everyone can make great art, you just have to keep at it!)