I just feel so scared and even the thought of being alone is scary
It is scary when you've never done it before. When I left my husband in 2005 at the age of 32, it was the first time I'd EVER lived alone. I'd gone from my mum's to living with a boyfriend, back to my mum's, living with different boyfriend, back to mum's, living with husband.
Even though our marriage was fucked, I put off leaving for ages with stupid fears like:
Who will get rid of spiders?
What happens when my PC breaks down?
How will I get heavy items moved around?
What if a menacing looking man knocks on the door late at night?
They were not real reasons for me not to leave, they were just convenient hooks for me to hang my reluctance to change on.
(The answers of course were:
Me, with a long extension nozzle on the vacuum
I will take it to the PC repair shop, or work out how to fix it myself
I will ask for heavy items to be delivered to a specific room, or for existing items I will ask friends or neighbours to help and repay them with dinner/wine/chocs/something in my skillset
I will call the police)
It sounds like your mum and stepdad are supportive. You are going to be absolutely fine.