My kid is a Posse Scholar, and went through the process last fall, and her experiences in every one of the interviews were all positive. In fact, every single one of the 10 students from her high school who participated in the first large group interview said how much fun it was, and how many cool people they met.
I just met a woman who is a Posse alumna, and she told me how she was very quiet and also, like you, very hesitant to talk in large groups, etc. As I wrote, she is an alumna - she was chosen, went to college ( a very selective one), and graduated. Posse are looking for all kinds of leaders, including, and especially, those who quietly lead by example, and by getting things done.
The first “large” group interview consists of multiple smaller group activities - the point is to see how you work in groups of 10 or so, not in a large noisy room full of kids. You will be surrounded by kids like yourself, who were chosen by their schools because their GCs or others believed that they had the potential to succeed in the Posse program.
In most ways, Posse is far superior to QB, because they don’t simply choose you and toss you into college. They prepare you for college, including things like teaching you how to choose classes, how to write at a college entry level, how to write a resume, or how get an internship. They create a support group of students who attend the college together, and they have groups of older and more experienced students who have been at that college for a while - the older Posse groups at that college. You also have a faculty mentor at the college who provides personal support and advice. Posse also helps after graduation with getting jobs, internships, and getting into grad school. Over 90% of Posse scholars graduate. This also means that Posse is very good at matching Posse scholars to the best college for them.
You write that you suffer from social anxiety, which means that going to a challenging college will be especially difficult for you, especially if you lack a support system. That is another reason to look at Posse.
So, while Questbridge and Posse are similar in getting you accepted to colleges (about 6% of QB applicants receive full scholarship versus about 5% Posse nominees), Posse is far better at getting you through college.
As for colleges - which Posse city are you at? You may not especially “like” any of the Posse College partners of your Posse city. However,
A, how can you even know whether you like a college, since it is unlikely that you have visited them, or actually have had direct, first hand experience at them? Most of your opinion may be based on limited and superficial information. I’m not criticizing, since that is how it is for the vast majority of the population. So there is a very good chance that you will actually find a college to love in that list of partners if you look at them with an open mind.
B, a full tuition scholarship is something that would make me, at least, love a college very much.
So definitely go to the interview, and do your best to be your best self and to be contributing part of your group. Find kindred spirits and, if nothing else, get contact info for some really smart and talented kids your age.