Welcome Immortal
Yes, it's painful ~ and almost everyone knows that, because almost everyone has been through it ~ unrequited love.
On the other hand, you say that it was
she who liked you first, but that by the time you showed an interest, she could only like you as a friend, because she had agreed to date someone else. And, of course, while she is dating him, it is only right that any relationship with another boy should be purely platonic.
In time, she may change her mind and choose you over him ~ so yes, who knows, you may indeed become the attraction one day ~ or maybe they will remain happy together.
Either way, you need to try to fill your time with interesting people and activities, but 'getting intoxicated' is not to be recommended.
If you want to stay in touch with her, you must make it clear to her that you are prepared to accept just friendship ~ even if you want more than that ~ and you must stick to the agreement.
Good luck