If you have slight astigmatism and can see fine without glasses, you don't need to wear them all the time. If you have a headache or blurry vision when not wearing glasses, you should wear them all the time otherwise your vision may get worse.
Yes. You need to wear glasses all the time if you have astigmatism because astigmatism will make things blurry if you don't wear glasses to correct them. If you have bad vision, you will get headaches if you don't wear glasses. I have astigmatism, too. My doctor advise me to wear glasses all the time, or my eyesight will get even worse. Now I wear my glasses all the time.
You can get contacts. I've been wearing RGP (hard) contact lenses for 2 years now. I couldn't stand the weight in the soft contacts, or having to wear glasses all the time. They're pretty uncomfortable for 2 weeks but I'm so happy I made the switch.