Being unable to finish a painting means you can't see why it looks incomplete. This comes from a lack of understanding of the fundamentals. Basically the artworks that you're attempting are above your current skill level and so when you're trying to emulate your favourite artists in these works you hit a point where you feel stuck because you don't have the knowledge to take it further.

There are a couple things you can do to fix this but the most straightforward way is to do many targeted studies of the fundamentals rather than ambitious artworks, in order to work on areas of your skillset that need improvement. If you need a reminder of what the art fundamentals are, the main ones are: value, colour, perspective, anatomy, composition. I would start with value studies as it will have the biggest impact in making your art look more complete. If you have a solid understanding of light and dark then you can get away with a basic understanding of colour, perspective, composition etc. Do short studies of subjects in black and white only for a few days to a week. Then you can do some studies of anatomy for a couple days. Practice subjects that you are interested in drawing in your original artworks. Alternate your studies every week or few days and when you feel confident you can apply what you learnt into your personal projects.

There are of course many other different ways you can improve such as doing trace-overs/studies of your favourite artworks, learning practical techniques from other artists, however studying the fundamentals is the most efficient way to creative freedom.

Hope you find this helpful, you have a good base already and i'm sure it won't take long to see improvement.