Why is it that I understand complicated subject matters but not simple ones?

You need to put it into a context so it will make sense to you. Right now you have it written off as useless so your brain won't retain the information nor work out the problems. How is it useful to you to know this stuff? How do the bonds of a gene connect, what are the angles, why should they be at those points, there is a reason, and if you can calculate the relationship and distance between then you gain understanding of both their functions, why some are turned on and how they can affect others. A context for learning, a reason to know and use this.