That's not an externality. That's what you pay for.
Other people, not just you, also benefit from your increase in flexibility.
Do they? I guess occasionally my family and a few friends benefit when we can meet somewhere they haven't bothered to support with proper public transport. I'm not sure I'd class that as an externality.
It's not the most convenient because of cars. It's more that the other options are less convenient because of cars.