Tips to get better?

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

I did some corrections. There are a lot to fix, I'm not going to get into details because these subjects can't be covered in a comment. The reason you can't get the painting right is not the overly smooth texture but lacking information on value range.
You have one idea to describe the form and you apply that for everything, light is not consistent, shadows are not consistent. If you have a sharpe edged shadow somewhere, that means you have direct sunlight falling on the subject and you gotta apply that everywhere. You also mixed the styles, the flowers are flat with black outline, the guy is realistic, etc, but I assume you got impatient with painting the flowers.
White is a tricky color because it almost never occurs naturally, unless your object has a white base color. Only the highlight on glossy objects and liquids are gonna get white, in this case the skin oil is causing reflection, so you don't have a skintone to white gradient, you have a soft midtone gradient with white dots on top. In case you still see this effect on photos that is because they manipulate it, or when the contrast is high the camera burns the light out resulting in a white patch ( cameras can't capture natural value range for both shadows and light )
Skintone has a lot of hue shift caused by it's transparency and bounce light from the environment.
Youtube videos won't help you get better as they are bite sized lessons only scratching the surface, I recommend to take Sam Nielson's online class on schoolism, its called Advanced Light. James Gurney - Color and Light is a very good book on the subject. Ctrl + Paint video library also covers many principles and its free