On the one hand, sometimes you have to fake it until you make it. That is, many of us experience impostor syndrome at some point in our careers. We spend so much time worrying that others will discover our shame, that we don't take a moment to realize that everybody else is also faking it until they make it, and so have little time to devote to uncovering others' alleged shortcomings.

I really want to be good at what I do and studied really hard to be a C/B average student in college

On the other hand, maybe you aren't cut out to be an engineer. While grades are by no means the only, or even the principle, indicator of one's potential success as an engineer, your self-admitted struggles in college to be merely a C/B average student could be an indication that some concepts and skills necessary to be an engineer are beyond your current ability. There is no shame in this, per se, as each of us have different abilities and aptitudes, which abilities and aptitudes we discover and uncover as we journey through life.

Perhaps you could seek out a mentor at work, an experienced, trusted senior colleague, who may be willing to talk with you about your concerns, and offer guidance and advice.

It may be helpful to engage in critical personal reflection, and make some lists of your strengths, things that could be strengths with additional training/education, and things that are weaknesses and never likely to become strengths.

Perhaps you might find a different type of engineering work, one where your strengths will be valued, and your weaknesses will not be a liability.

Whatever you choose, and whatever the outcome, good luck!