I am in a similar boat. I am a senior who is applying to graduate school, and I often feel too dumb to go. I am taking science classes where I feel I am the stupidest person in the class. During lab, I often have literally no idea what is going, while everyone else seems to know exactly what they are doing. I have to try incredibly hard just to barely pass, while others are able to ace the exams with just a nightsworth of studying. Will I still apply to graduate school? Yes. Because graduate school is not a measure of how well you do in your classes, it is a measure of how much you want to do what YOU want to do. For instance, my background in science is sub par, but I am a great writer. So I will apply to enivornmental policy/ethics programs by advertising my strengths with writing, and add the science background as a bonus. Find your strength, find what interests you about them, explore what you can do with them, and find a grad program that is appropriate for you. Graduate school in straight science like biology or chemistry? I'd fail and drop out. Something with more writing? Yeah definitely.