In my experience, there is no constant learning curve even if you study regularly. Getting better comes in incalculable spurts and sometimes you have to wait a long time. Be patient and confident with yourself.
You can see if you've gotten better by comparing your old sketches with the new ones. Don't compare yourself to yourself or others every day. It's just frustrating. If you are frustrated, take a step back and draw something simple that is fun and you always succeed (maybe doodles?)
If you're working on a picture that's important to you, take regular breaks. Breaks in which you are going away from the workplace and doing something else.
Think about how many hours you can really and joyfully concentrate on drawing. What I mean by that: I'm a full time author and I write 1000 to 1300 words every day 4 days a week. The rest of the days I do other publishing-related stuff. It took me a while to realize that this work rhythm is perfect for me.
Find your own work rhythm. There is no universal one for everyone. People are different.