A community mental health clinic, like Angelique said, could be a good place to start. If your school has a counsellor, I would suggest talking to them.

Being 18 and in your senior year of high school, it is completely normal to feel stressed and anxious. Remember, there's nothing inherently wrong with these feelings, but it does sound like it's taking a toll on your mental and physical health. And I know that sucks, because it can cause even more stress to know your health isn't in good shape. This can create kind of a messy domino effect. Just know that you CAN take control of your situation and return to good health. If local clinics or school counsellors are not an option, you could read up on some anxiety-reduction techniques. Most techniques are fairly simple and do not require much time or energy to do. PsychCentral has some articles that may help (e.g., 9 Ways to Reduce Anxiety Right Here, Right Now | Psych Central). And there's plenty of other sites or books that offer similar strategies. I personally find mindfulness meditation and physical exercise to help with my anxiety and panic attacks.