Can I start a new return instead of using the original one? I've made too many mistakes & want to start a new one. But it would leave an unfiled return in my account.

Yes.  If you haven't filed, registered or paid you can try to clear and start over

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-clear-and-start-over-in-turbotax-online/00/26444

Or if it doesn't let you clear and you haven't paid and filed you could just start a new account.  Online is only good for 1 return per account.  But you can use the same email address for 5 accounts.  Start an Online return here,

https://turbotax.intuit.com/personal-taxes/online/

in List of Forms, delete or zero out all the forms you entered.

That is essentially starting over.

@btrflygirl123-ao