A couple of different suggestions:
USE A SMALLER BRUSH. If you think your brush is too small, go smaller. I used to use something like a size 64 brush on a 14″x16″ 300px canvas. In retrospect, that’s HUGE – now I use something like a size 20 or 30, sometimes smaller if I’m working on the face. I definitely use a smaller brush if I’m zoomed in on a part of the canvas. Compare: My old brush sizes versus my new brush sizes. Of course, there’s nothing wrong with thick outlines in artwork, it’s all a stylistic preference, but I’d add those in later.
Zoom in! If you’re working on the face, zoom in on the face! Don’t try and work on the whole drawing at 100% all the time. I like to strike a balance by zooming in just enough to see the entire part of what I’m working on – the face, or the eyes, or the arm, or the torso or whatever.
Practice! Especially if you’re using a tablet and not a Cintiq (though I imagine there’s still a learning curve for the latter). The motions and muscle movement in drawing on a tablet is HUGELY different from traditional drawing, because you’re making movements that are disconnected from what you’re seeing – you’re not drawing on a piece of paper where the marks are right in front of you, your arm is off to the side while you’re facing forward looking at a digital recreation of the marks you’re making. I mean, try drawing that way traditionally – put your piece of paper a foot off to the side and stretch out your arm to draw that way. I bet you’ll come up with similar results.
Ditch the creative drawing stuff for a bit – work from life. It’s a lot easier to recreate something you see in real-time, in my experience – I started drawing portraits digitally because I didn’t have to try and focus on recreating an image in my head, on my tablet, which was disconnected from the screen I was viewing it on…it helps you practice the physical motor skills that you need to develop (rather than “mind’s eye” and technical knowledge that most artists are trying to improve on). Focus on doing exercises that fine-tune those motor skills at first instead of creative ones.
At the end of the day, you may always prefer traditional drawing – I still have a huge preference for it, even though I do most of my work digitally now to save time and money. I just like the feeling of pencil on paper.
If that’s the case, don’t be afraid to do your sketch and line work traditionally and then scan it and color it in PhotoShop! I’ve done that before and it comes up with some pretty nice results.
It takes practice to learn any new medium – if you picked up oil painting tomorrow, there’d be a learning curve there, too. The good news is that you pick up the technical aspects of a medium a lot faster in my experience than the creative ones (i.e. proportions, values, etc.), so give it a few months of practice and see where you’re at.