I'm thinking about cheating on some upcoming tests to get good grades
Is cheating something you do often?
Yes, I'm always cheating on my family, cheating my girlfriend by lying to her, cheating myself because I'm not good enough to pass the test.
Cheating is wrong and can lead to serious consequences. Instead of falling into habits of cheating in different areas of life, you should try to practice more integrity by studying harder and being kinder to your family and girlfriend.
I'm good at cheating.
So far you haven't been caught at what you are doing, but that does not make it acceptable.  As you said, you are cheating yourself by passing tests without knowing the material, and this can hurt you later on when you need that knowledge.  Additionally, your girlfriend could leave you and your family will lose trust in you when they inevitably find out.