So here's my three things to improve your painting:
1. Make a "Little" Plan
I do mean little. Whether you call it a thumbnail, a notan, a value study, it doesn't really matter. What matters is taking a few minutes to work out your composition and value plan ahead of time. This doesn't mean you stay slavishly to this plan if something else wonderful arises, it just means you'll likely not get caught by an unresolved composition which will just be magnified in the larger version.
2. Be Abundant
Yes, our materials are expensive. So is food. So is everything. I do know that if I worry about wasting, I'll waste. I will be stingy in painting and the painting will be stingy. So, I paint like the proverbial billionaire and try not to worry about it too much.
3. Be Compassionate
I try to be kind to myself and realize that not every day is going to produce a "winner", not even every other day or even every week. That's just part of the process and the "bad" ones are not just part of it but a necessary part of it. Love the bad ones as much as the "winners". You can't always be up or you wouldn't know what down was!
As Always, happy painting!