Here are a few important things which are hard for me to do. I often need to force myself, but it is always worth it.
...go back and fix if there's something which bothers me. Sometimes a few strokes are all that's needed. If I've spent 2hrs on the design, taking 10 minutes to correct the mistakes / little things which still bother me is totally worth it. If I post online it can feel good knowing that the [too short arm] or whatever has been fixed. If I don't fix, then I feel ashamed every time I see the image...
...kill my darling. Sometimes I have a good detail, or a detail I think is good, but it still bothers me. Perhaps it just doesn't work in the context with the other stuff that's going on. also, just because I've spent time on something doesn't mean it's somehow increasing the quality. I'm just a mammal making marks on a board.
...think about the greater good. If I am designing a set of characters, they need a certain coherence. This means that I sometimes have to change an great design to something less great just to make it fit in. I always try to remember that I'm designing a set, not individual characters. It's like the entire character set is a single large image. Every character can't be a blinged out alpha male. Also, If I need to keep physiology consistent I often imagine an ancestor, then I evolve it into several sets of monsters. The same can be done with armour, weapons, etc.
...remove the unnecessary. If a detail or line does not contribute it must die or change into something meaningful or quiet. It is easy to add but difficult to take away. People will look at the details and structures and try to make sense of them. I try to go over my designs and figure out which details are just meaningless fillers. If I have a focus area with a meaningless detail taking up space, I try to change it into something functional or figurative. Areas which are not in focus should not attract attention with loud nonsense details. A good way to quiet them down is to just brutally remove stuff, or make sure the details are quiet and pleasingly decorative, flowing with the rest of the design. If they don't flow, they'll stick out and attract too much attention.
Often when designing stuff like spaceships which have 'greebles' (random tech stuff) I start doing too much meaningless noise, and then I have to back away and figure out how to get a good silhouette and suggest a meaningful structure of the tech details. An analogy could be music where you have a theme, certain instruments, an overall shape, and also rhythm in the detailed parts. BAM BAM BAM ... dirudiru-lutt. Whatever. Not F7GrkFd5gsSvjksRg0krt.
...throw away the first sketches and iterate the design. Sometimes a design comes out pretty good right away, but I've found that redrawing it makes it much more fluid because I've learned how to draw it by heart. It doesn't take much time to make another rough. I won't know if it's actually strong until I've let it do battle with a few other variants.
...flip/mirror, even if I can't see any problems. Every time I flip the problems show themselves, and I'm surprised every time because I thought it looked okay before. I think my eyes are a little skew.
...try to stay reasonably faithful to the source material when doing designs based off old game characters. It is possible to stay faithful to the source AND do a great design. What's the point of basing a design off something if it ends up looking nothing like the original? I try not to make arbitrary details when I just as easily can use something from the source and increase likeness. If I can't manage to do that then I'm incompetent and only capable of simple, blind, low vocabulary reflex drawing.
What is realism? When a camera captures photons bouncing off me, is it capturing the real me? Ultimately, you will probably be viewing a 24-bit image of me where the dark and bright colors have been cropped off. One day we can probably build a really advanced camera which captures all kinds of light, radiation and emissions. Is this enough for a realistic portrayal? It's a question of semantics I suppose. A photo is "photo real", perhaps like the senses of a slug are "slug real".
The representation of me in my brain feels real to me. A photograph of me feels fake and superficial. When I look at other people, I don't just see the light reflected off their bodies. I see what mood they are in because of something which happened earlier, and a movement they are about to make. I could be focused on very subtle changes in their face. I see things which a single frame wouldn't capture. There are also things which I don't see, such as what mood a person is actually in.
I can imagine that back in the days when the camera was invented, some superstitious people may have thought that a camera would capture your soul. I don't think that a camera does that, but I think that, perhaps, art can. Well, not in an evil way I hope.
There's anime where the characters suddenly become super deformed with exaggerated emotions. Is this unrealistic, or an effective way to show their true emotional state better? There's also the cartoons where someone is bumped on the head and gets a huge swelling. A bump on the head probably won't show up clearly on photos, but it can certainly feel huge. So called unrealistic art can portray things which feels very real to us.
Even if you set out to do a "photo realistic" painting for some reason, it's possible that it will still look off even though you followed the reference material slavishly. Perhaps the photos which you used were badly taken. Maybe the person in the photo has really small hands or an asymmetrical face. Our eyes and brain manipulates the images which we see in various ways. A white paper on a table before us looks white and square. A photograph will probably give the paper a blatant yellow tint and a fisheye curve. So, what we see and memorize as real is not necessarily that close to what's out there.
If you're attempting to paint a generic pretty woman photo realistically, then it's quite possible that you'll have to play Frankenstein with some parts to create that feeling of photo realism. People who see the painting have a certain... normalized idea of a human which they expect to see.
In conclusion, I think that, to excuse the things which looks wrong on your painting with "that's how it looks on the reference" is to do yourself a huge disservice as an artist.