"Glue is just as good as sewing, right?"
It will definitely depend on the type of glue that you use! If this is a quick costume that you're not going to wear again, go for the hot glue. If this is your entry to the next season of "Project Runway," you need to make it work with thread.
"Is it too late to buy a costume?"
If you're willing to pay triple the amount that costume costs, then no. If not, keep sewing.
"How will I ever get this glitter/string/yarn/paint off my floor?"
Look at you pulling out all the stops for your costume! You're going to be seeing the remnants of it on the bottom of your feet for a long time.
"How easy would it be if I just went as a sexy cat?"
You're going to look like half the parade of other "sexy fill-in-the-blanks." Keep going! You're almost there!
"Ughhhhh! I have to wear costume makeup for this — don't I?"
If it's going to add to the overall look, then, yes. If not, then nix it. Good thing, though, we live in a world where we have an endless stream of DIY makeup tutorials!
"How am I going to use the bathroom with this on?"
Sometimes our blind ambition inhibits us from seeing problem spots. Do a bathroom run before debuting this piece out at the bar so you're not walking around all night with a full bladder of whatever drink special you had.
"This is close enough, right?"
One hour ago you were going to call it quits, but now this looks good enough to nonchalantly say to your friends, "Oh yeah. I made this. NBD."?!
"Am I going to be cold?"
You're not about to hide this awesome DIY costume with coat. Drink a hot toddy and go for it.