Bro, girls aren't robots that'll dismiss non-perfect attempts, they're humans just like you. Suppose that this girl was feeling the exact same way as you are, which is entirely possible. What if she were to ask you in a clumsy manner? Would you reject that? Probably not. Unless you're in the middle of brawling a bear with your bare hands, it's not the wrong time. Unless you're in the middle of a funeral, it's not the wrong mood. Your face will do its jazz on its own, because most facial expressions are subconscious. As for the text? That's not very important. It's not WHAT you say, but HOW you say it. And if you're sincerely trying to express your feelings, you're saying it the right way, even if you're just garbling out something. There's no single definite right way to tell someone how you feel about them, so don't worry too much about that.
Also, for what it's worth, i don't believe that you suck. I mean, you had the cojones to ask us for help, right? I know it's easier on text, but remember, it's still humans on the other side of the monitor, just like the girl you want to talk to. It's not illegal to think good things about yourself in your head, it's in fact encouraged (within reasonable limits of course) and beneficial (higher sense of self-worth). It is very important to feel good about yourself, because remember, you're the one who has to live with yourself forever.
One thing you can try to get away from the toxic mindset of "i suck :c" is to have a little journal for yourself. Try to write in it daily, but don't sweat it if you miss a day or two. And on top of usual journal things, try listing down 3 good things about yourself or good things that happened to you every time. Can't remember who it was, but it was some awesome person on this forum that has mentioned this tactic more than once. It might feel awkward at first, but trust me, it'll get nice quickly. It's also nice to have a place where you can dump all the feelings you have that you don't wish to share with the entire world.
Edit: VECTOR IT WAS. Our lovely mathematical phenomenon. <3