Maybe she just sees you as a friend. The way you two talk doesn't necessarily mean she's looking for something more. If you're certain she’s already seeing someone, I wouldn't try anything. Even if people say you can change your mind at the altar, why risk it? Your only real shot is if you happen to be lucky enough to be more than just a buddy to her.
I deal with OCD too, though maybe not quite as extreme as someone needing to check gas valves, water taps, or locking every door. Still, I can’t help avoiding the grout lines between tiles. Like, back when I was working in Cleveland walking along the waterfront, I felt this intense urge to touch everything an even number of times. Or if I’m hammering a nail, I feel like I have to strike it an even number of times.