I wish I knew how difficult the transition from undergrad to grad school would be. Now that I am in my second semester, I am over the transition "hump" and am feeling more confident.
Once you find a lab and start a project, you are going to find out how much you do not know. Be ready for a humbling experience.
Your self-talk matters. During this process and maybe during graduate school, you will have ample opportunity to beat yourself down. When you notice this negative self-talk, try to replace it with forgiveness and kindness. Most people believe that this will enable their negative qualities, but research shows that this is not the case. I used to think this phrase to myself when I felt down: "Today wasn't so great. I could have done more. I shouldn't have done (something), but tomorrow is a new day. Tomorrow I'm going to do better." It helped a lot to actually have a more effective next day. I also felt better, and that's invaluable during a stressful time.
Graduate school: Everyone is feeling like an impostor. It is very lonely. Find friends and cultivate your friendship. Don’t compare yourself to others, we are all in different places in life.
When lab work gets difficult and nothing seems to be going right (which happens all the time in Grad school), having a reason why you are there may be the only thing that gets you to the end.
This a system designed the best that it could be to prepare you to be a scientist/expert in your field. Trust the process, put in the hours and things will come when they should. Don't get complacent, but don't be an overachiever because when you're on the front lines failure happens more frequently.
I would have taken more breaks. When progress stops and your mind is embarking on tangent after tangent, a break can be really useful. The type of break that is useful: make a snack, take a walk, take a nap (20 min. or less), do some exercise, clean a little, etc. Try not to watch TV or do activities which you find distracting or addicting. You don't want to lose focus.
While reading papers upon papers may not be the most exciting thing for most, it is crucial for staying up to date in your field. I suggest instead of reading every paper in its entirety, like I did when I first started, read the abstract, intro, and conclusion. This should give you an idea about the findings of the paper and if it's relevant to what you're researching. This will save a lot of time on reading and you find the paper is relevant then you can read it in its entirety. Also, reading reviews is useful for getting background information on a topic, but be sure to read the papers that the review cites, as that's where the details are.