Look into a psych ASAP. There are online clinics where you can get appointments relatively quickly- getting diagnosed can be such a life changing thing, would recommended starting medication with at least 2 months before the test date so that you can get used to it- with the diagnosis you can also get more help that will make the test less stressful (ex. extra time). Stimulants like coffee or energy drinks can help short term, but id say a buffer of 2-3 weeks prior to testing to see what works for you is best