It sounds as though you might be approaching the art of sketching as figure drawing or still life drawing.
Instead, your sketches should be a visual interpretation of your ideas - the goal early in a development process isn’t to mimic real life, but to offer visual communication of scenarios, options, styles, features, etc in a loose & quick manner. For the designer, these early sketches offer an opportunity to explore forms, transitions, emotional qualities, etc.
There are a good number of VizComm and ID books to reference - and while some might seem outdated at first glance (‘gouache renderings for industry’, ‘rendering in mixed media’, ‘creative marker techniques’) they are all excellent for reference, as the goal of a sketch and its appearance is the same no matter the media you use (mostly digital these days).
Exaggerate your focus areas, leave certain parts and details to the imagination, accentuate your contrasts, etc…make them pop off the page, make them fun and lively. I’ve inserted a few old sketches below for reference.