Do you all think skipping 11th grade will benefit or negatively affect me?
I'm currently in 10th grade with a 3.9 GPA but I have 16.5 credits and I will be going straight into 12th grade next year. I'm kind of scared that because I didn't have as much time to build my extracurriculars, colleges may deny me.
Skipping a grade can put you at a disadvantage unfortunately. You'll be applying to schools against kids who have had 4 years of HS to build up their grades, their ECs, all that. You're going to want to start working on ECs as much as possible now if you do end up skipping the grade. Something to also consider is how this might affect you once you're in college. I doubt being a year younger than people will be too difficult to deal with but if you're shy in general it might make things harder than they need to be.
Is this something you have to do or can you just not skip? If it's something that you have to do I'd encourage you to look into a gap year once you graduate. You have plenty of time to come up with a plan to make the most of the gap year which would let you build your EC profile, hopefully provide you with unique experiences, and let you catch up in age to everyone else.
All-in-all though, try not to stress too much. You can (and will!) still be accepted by schools if you decide to skip and apply next year. It's in no way something that will set you back in huge ways, especially if you can work on those ECs now. It might even give you something to write about for your essays!