Hi Youngperson183 and warm welcome to our forums
Turning 18 is a big thing. There are so many changes in your life - as you've pointed out above. I'm not a health professional, just someone who lives with anxiety and depression.
Change can bring on my anxiety. Maybe that's happening to you. Those a big life changes you are going through:
no longer having support from your parents
lack of things to do (e.g. studying for exams)
boyfriend going away
All these types of situations can set of our anxiety. I know it does mine. The changes in my life set of my anxiety significantly - retired recently and therefore have no long term prospects for an income (other than the pension eventually), reducing spending after a lifetime of a wage coming in, inactive mind that has been working for 50 years). So I do understand how you are feeling.
Do you have anyone you can talk to? For example a close trusted friend or family member. Someone who will understand how you're feeling. Since you've been diagnosed with GAD, have you been back to your doctor to let them know what's happening? Do you see a health professional, e.g. a psychologist? I find my psychologist is someone to go to when I feel my anxiety starts to get out of control.
Something else to think about is - there are lots of self help tips throughout our forums. Have you browsed or searched through these yet? There are some good stories shared by people who experience similar things to yourself.
What sorts of things interest you, e.g. sports, movies, books?