Is a girl's intense love for another girl—to the extent that it affects her devotion in prayer, makes her constantly think of her, fear her, and blush when speaking with her—considered polytheism (shirk) and taking an equal to Allah? And is it possible to repent from this sin?

Satan is an enemy to man and seeks to mislead him and turn him away from guidance. Allah Almighty said: "Indeed, Satan is an enemy to you; so take him as an enemy." He approaches man by beautifying sins, step by step. What you feel towards this girl is from Satan's beautification, as he began with admiration, then attachment, then عشق (passionate love), and he beautified it for you as "love for the sake of Allah" to justify this forbidden matter that has affected your obedience. Your duty is to offer sincere repentance and avoid this girl. For the treatment and ruling concerning عشق (passionate love), you can refer to Fatwas no. 34504 and 81904.