How should one act concerning loving a girl while currently unable to marry?

The best solution for your problem is to marry this girl, as the Prophet, peace and blessings be upon him, said: "Nothing like marriage is seen for two who love one another." If marriage is not possible due to financial inability, or the disapproval of the girl or her guardian, then you must sever the relationship with her and stay away from her so that love does not turn into passionate adoration, which is a malady difficult to cure.