Resonance structures are determinable in that they allow us to make and test specific predictions about a molecule's structure and properties. For example, in an amide, you can draw a resonance structure where the double bond occurs between the oxygen and nitrogen instead of the oxygen and carbon. From this resonance structure, we can infer that rotation about the C-N bond will not occur because of the partial double-bond character of the bond. This has been shown to be true.
As for resonance structures, its important to remember that resonance structures don't actually exist. The structure of a molecule will be the superposition of the molecule's resonance structures (the resonance hybrid).
The concept of resonance structures is helpful for introducing the concept of conjugated pi system
. However, in more advanced chemistry, I believe it is more useful to think of conjugated pi systems instead of resonance structures.
Anyway, don't feel too bad about not being able to fully grasp everything in chemistry. Chemistry takes a long time to learn. I've been studying chemistry for much longer than you and sometimes it seems like certain basic concepts still don't make sense to me at all
But, that's what makes chemistry fun. There's always more cool stuff you can learn.