Possibility of Changing Bad Impressions
The truth is, even though people don't like to talk about it, that most graduate students start out like you do. As advisers we see it all the time, and the better ones among us have conversations about these sort of things with our students and/or to our research groups. Many students, and not a few of those who later make it into the professoriate, have at times struggled for weeks or months with frustration, depression, and general unhappiness with their progress in graduate school.

In other words, what you're describing is, if not a normal, a common experience in graduate school, even among some of the best students. I like to thing of graduate school as less of a way to filter out the best than a way to filter out those who can deal with frustration and yet persevere with their research.

So keep going on. If you want, seek out the postdocs and/or your adviser to talk about this. You'll find that you're by far not the only one to encounter such feelings.