Thanks so much for reaching out to us tonight! It sounds like you're going through a tough time. School is hard for a lot of people, you are definitely not alone! Junior year can be especially tough because that's the time when people are focusing on colleges and taking a lot of the tougher classes than freshman and sophomore year (and senior year when people generally tend to slack off).
So its not uncommon that some classes might be a struggle for ya. Have you tried talking to the teachers of your classes and seeing if there is any extra credit work that you would be able to do to make up your grade a little bit? Your teachers most likely don't want you to fail, and will try to help you if they can. It might also be good to talk to them and see if they would be able to tutor you a little before or after school, or pair you with another student that can tutor you.
It might also help to get a study group together. Talk to some of your friends or peers that are in your classes and see if they want to get together a couple times a week to study and work on homework. Sometimes it can help to hear others' perspectives on class notes/information, plus it will help to hold you guys accountable to make sure all the assignments get done.
As for the ACT, have you tried taking an ACT practice test? A lot of times school counselors have books in their offices that teach you how to prepare and study for the ACT, and there are online practice tests you can take too. There not a lot of fun, but they do help to prepare you for what its like. Ultimately, just have confidence when you go in to take the exam. A lot of people take the ACT over again for a better score--most people are not good at standardized tests, so two or three tries at it is not uncommon.
Its good that you have high goals about getting into college, and have set your sights on a particular one. Don't lose sight of that goal, and don't throw the towel in just yet. You still have a whole summer that you can prep and work on applying to college, plus it could be that you take a class or two in the summer (summer school) to bring up your GPA, or even apply during your senior year.
If you have to go to community college for a year, its okay--you can still transfer to the college you want to go to the following year (if you still want to go there)--so its not completely out of the picture. It sounds like you might put a lot of pressure on yourself. Take a breath. Allow yourself some time to relax. Its going to be alright.
Colleges take a lot into account--extra curriculars, academic classes taken, ACTs, and the application essay (that's why they have you write it). Some people take all the easy classes so their GPA is super high--other kids work at it and take the tough classes, which is naturally going to lower a GPA. Colleges account for that.
Keep trying and do your best! Use your resources and you can definitely bring up your grades. Let yourself relax though, because the stress will just continue to overwhelm you and make it harder to focus. Take a breath now and then and remind yourself that you can do it! Hang in there! Hopefully, it will get a little bit better for ya!