"I have been accepted into an amazing clinical psychology program in a top school."
Congratulations! That is great to hear! Also, have you been provided information regarding funding security?
"My main concern is that I am coming straight out of undergrad (I will be graduating this spring). While I have a lot of undergrad experience, a few publications and presentations and an honors thesis, I still feel that I am nowhere near being qualified enough for this program."
It may seem hard to believe, but for INTERNSHIP applicants, the average number of publications (with any author order) is ZERO. As such, you are likely way ahead of the boat in that department, and likely what made you appear as a great candidate.
"I see everyone has so much post-graduation experience,"
See how? Did they tell you, or are you making assumptions? Clinical or research experience? Again, two completely different things.
"I feel like I will miserably fail the second I get to this program because of my lack of experience."
One thing you will be trained at very quickly is to challenge automatic negative thoughts, which you have a perfect example of with the statement you made. It sounds like you feel anxious and concerned about failing miserably (outcome not feeling) because of your lack of experience. With what you said about your research background, I don't think you need to feel that way with regard to research. Clinical experience is difference, and other individuals in your program may appear to be able to more quickly display empathy/rapport building skills. However, as long as you have basic social skills, are motivated, caring, and willing to listen and communicate, they will develop with time. Everyone will be evaluated and receive critiques, which is normal and should provide guidance for developing skills as a clinician. Together, your program would not have given you an offer if they did not think you had the research or social skills needed to be an effective clinician.
Also, the feelings and thoughts you have are completely normal, but with time you'll come to see them dissipate. I also entered my program out of undergrad (5th year now), and have had similar worries be struck down🙂