What are you drawing with - pencil?
I eventually got the hang of drawing as an adult, after attending (many) art classes which teach drawing with charcoal/chalk, not pencil. A large part of drawing is actually observation, not mark-making. In reality there is rarely a visible thin line delineating one object from another, which is why it's difficult to draw a representation of real life with pencil.
If you really look at an object, you don't see a line outlining each element, you see blocks of colour, which change in tone as light falls on the different areas of the object, giving it visible form and shape. So when you draw with charcoal, you don't draw lines (well, a few light ones to start with, to block where everything goes), you use the side of the charcoal to create the 'blocks' of different tones, which eventually form the whole object. (You can also smudge out charcoal to reposition your drawing as you go along. It's a great tool for learning but not for producing lasting pictures.)
Learning to "draw what you see" not "draw what you know is there" is also important. Again, that's partly about there not being a 'line' between objects, but also not thinking about each object individually, but seeing everything as a mish-mash of shapes next to each other and how they relate to each other. The concept of negative space is very important - it's the 'empty' space we think is unimportant because there's nothing solid to draw. But it's a shape in itself and learning to look and draw it accurately helps to put all the shapes of the solid objects in the right place and make the drawing look realistic.
For example, on my desk there's a mug, some Post-Its and a pen. If I were to "draw what I know is there" I'd probably start with the mug: curved circle for the top, two sides joined at the bottom to form the base, and a handle. Post-Its would be rectangles, partly hidden by the mug. Pen on top.
But if I observe my mini still life, and give equal thought to all the shapes and not just the ones that form solid objects, I see that the hole formed by the mug handle is a distinct shape, and through it I can see a corner of the Post-Its, which splits the handle hole into two distinct shapes: a small section of the Post-Its and a larger section of the table top. If I get the placement of the corner of the Post-Its in the right place in relation to where it appears through the empty space of the handle, it will help me place the rest of the Post-It notes protruding from behind the mug. But the shape of the table top seen through the handle is just as important as the solid object, and sometimes the shape of negative space between parts of objects is more distinctive than the solid object, and drawing the negative space helps to get the placement right.
Shadow is also important, but I've already written an essay!