When you sit down to draw or paint a tree, it can be tempting to create only from memory. While this can work if your mind is able to see a three dimensional type structure, it can also end up looking two flat and cartoonish. In the paintings below, I used a photo for the sketch of the first two trees, and used my imagination for the third.
I like all three of these trees, as they have some interesting overlap and variety in the branches. It has taken practice to get the branches crisscrossing enough in an imaginary tree. I still find that I can get overly repetitive in my work, particularly when I'm tired or distracted. Being "inconsistent" in the shapes can be more difficult than being overly symmetrical!
You might find that you're making your trees into lollipop shapes, or triangles, or just that they all look the same. Go for a walk and study some trees, particularly in the winter when the branches are bare, to get a sense of what they really look like. Take some photos to draw from later. Or sit out in nature with a pencil or whatever, and just sketch some trees for practice.
As you practice, focus on the shape of the branches as this is what tends to give trees their personalities. They are so unique and interesting!