Timeline: Willow and Tara made up, but Tara didn't get shot.
You're stupid. Stupid, delusional, and at least a little self-centered to have believed it was at all about you. You should have known better. How many times do you have to be reminded that you don't matter? Nobody else ever really cared about you, as yourself and/or as a human being. So why should she have been any different? It's not like she didn't give you enough hints. But she made you feel loved. Hell, cherished even. So you let yourself think that somebody as wonderful as she is could love you.
Add ignorant and blind to the list. It's not like she didn't drop enough hints. Saying how cute you'd look with red highlights, asking if you'd ever considered green contacts, and isn't the pink fuzzy sweater she bought you the coolest? How much more obvious did she have to be? She did everything but come right out and say it.
Maybe deep down you knew. Maybe subconsciously you knew that you weren't good enough. Maybe that's why you never brought her home until now, on the night she was supposed to be out with Tara, doing something that you're not supposed to know about because you're 16, and therefore to young to know about anything that they don't teach in school. How could you have known that she'd be home because they had another fight about magic? It just isn't fair that when you finally get the house to yourself and you both come in giggling, and slightly tipsy, Willow's there and looking to rebound with your girlfriend who is apparently her ex, a fact of which you had no idea.
But the greeting she gives Cordelia is anything but subtle, all but jumping her the moment the awkward silence is over.
You're stupid, delusional, ignorant, and blind. You should have known. And now you do. Because from where you stand, you can see it all too clearly.