I think a lot of people feel this way (look up imposter syndrome). Now, I'm willing to accept that maybe you just aren't capable, but you're probably selling yourself short. I'm sure you don't want to ask your PI pointblank "do you think I'm good enough for grad school" but maybe you could subtly ask some grad students you work with what they think of you/how they knew they were ready for grad school.
When I was in undergrad, I would constantly mess up in lab and have to go to my PI and explain my stupid mistakes. I thought he considered me a huge dumbass, but in a letter of recommendation he wrote for me, he listed it as a strength! It's better to know when you're in over your head and get help than to soldier on into the abyss and make things worse. Messing up and failure are just a part of learning. Seriously.