When I was starting painting, almost every tutorial warned against so called "plastic look" and I think that is the biggest problem of your paintings: the subject is there, drawn well, but the whole painting just looks too digital (= unnatural).
The tree is very strange, the highly realistic foliage brush with the vivid green color looks completely out of place with the pencil-drawn texture of the tree bark. You used a black edge stroke on the little leaves on his chest, but the character is shaded completely smoothly without any stroke. The hair brush would look nice, but it doesn't have enough variety or volume to build the entire hair with it.
You worked more on the face - and it's definitely the best part of the image for me, it looks really nice.
The body would be good, even though I'm not a fan of the extremely smooth look, I've seen some artists pull it off - but the rest of the painting would have to match it.
The biggest advice I could give you (and yes, it's and old and typical advice, but very, very important) - try using only default pressure sensitive round brush for a while. Just that one brush. (Or, a simple textured brush.) Basically, try avoiding any "shortcut" brushes like leaves, hair, fur etc. and also the smooth airbrushes. Instead of constructing an image, try painting it. See if you like it. Maybe it won't be your thing and you will find a way to make this way work, but I would say it's worth a try! 
This is a very good article, especially number 9. for you, but all are good.
And this is a great video, actually, his entire channel is gold.
Well, that a long wall of text, I hope at least some of this will help you a bit
