Is there sin on me because of a girl's attachment to me and her admiration, given my awareness of that and my keenness to lower my gaze and not mix with her or speak to her, and is it incumbent upon me to do something to prevent her attachment and admiration, given my inability to marry currently?

A person is held accountable for their actions. If nothing has emanated from you, then there is no sin upon you. Marrying a religious girl of good character is a good thing if possible. Otherwise, you must avoid every path that brings you together, and work to instill despair of you in her heart by averting your gaze from her, and avoiding mixing or speaking with her.