Hey, @quizzy! I see what you’re going through, and I totally agree with @Yiming Wu - it’s a matter of shifting the mindset about how the work should be done. As Yiming says, a longer piece is almost never done in one sit, so splitting the process into several sessions can be a great idea. Start noticing how your own process works - when you notice you lose focus and the work isn’t flowing well anymore, that can be a good moment to take a break. But that doesn’t mean the work is finished, and that’s fine - after a while (minutes, hours, even days or weeks), it’s good to come back to it with fresh eyes and a rested mind to try to see more clearly what could be improved - then do another work session, then take more breaks as needed, and so on. That’s how most professionals do it. Along with that, try observing how you can break the process down into stages. For example: 1) PREPARATION - Usually, preparation work can come first: develop the idea, draw thumbnails to figure out the composition, research visual reference, maybe do some color studies or separate sketches for specific elements that will be in the piece. 2) ROUGH - Then, based on the thumbnails, doing a larger initial rough sketch might be a good following step. For me, this is usually done very intuitively and loosely, in a way that helps me materialize the idea as best as I can without having to worry too much about techniques or “correctness” just yet. 3) TIE-DOWN - After the rough sketch is done, I find it helpful to do another more careful sketch pass on top, and, this time, it’s all about making adjustments so it looks “technically right”. I give the piece a more analytical eye and try to apply all the techniques I know of to see how I can improve it. In other words, I’ll go over the whole piece again and re-draw it with the specific goal of fixing mistakes, making it better and getting the construction and composition as solid as I can.  4) FINISH - Once you’re happy with how the more careful sketch turned out, you can then start to put into practice your finishing/polishing process - and this could mean different things depending on the style and technique, but since you’re working with painting, it often means rendering. Rendering is usually the longest stage in a painting and, at least in my experience, it can take up to at least half the amount of total hours you’ll put into the piece, or sometimes even more. With some practice, these things combined might help you get to that mindset shift and even start to plan you art pieces as projects, not just sketches. Hope this helps. If you have any questions or would like to discuss this further, just let me know. Best of luck!

Humm I think you could just change the way you think how a piece is done. Do some more "realism" stuff (whatever that means)... Try to get a photo study to as close to the original as possible, if you are impatient after an hour, just do an hour, and get back and do another hour on top of it at the later of the day or the next day. because each time you get back to it you find mistakes and stuff, you can easily fix them. What kind of medium do yo use? If you are using watercolor then probably that's not very good for working too much. Gouache, acrylic or painting digitally could allow you to work over top of existing stuff until you get it right.