Most Helpful Girl
you're only 17. you still have time to find yourself.
Most Helpful Guy
So.. you're attractive, you hung out with a friend during summer
you're good good at art and acting (be confident about that!).
Even if you "only landed bad roles" a role is a role (job is a job)
there're many people who can't even find a job.
You claim to not have success with girls, but you said
that you've been on a date before, again, that's more
than what some guys have, even if she ignores you. ^.~
You claim that telling yourself how great you are is useless
since "you already know you're nice looking".
The thing is, being confident is not only about looks
and the point in tricking yourself into becoming confident
is not done in a day "know what, I'll be confident today"
What you need is patience, when attempting
the "tricking your brain" method.
It takes time to get your mind into the right mindset
small words, such as "I can do this" or "good work! [your name]
will actually greatly influence how your brain sees you.
You don't have to act confident, just be yourself
while complimenting yourself in your head/thoughts.
If you stick to it, you will see positive changes
sooner or later, that much I can promise.