I am honestly desperate

There is nothing I would love more than to be with this girl I have liked since 5th grade, I'm a senior in high school now so that is a long time to like one girl.
I have had two conversations with her. The first one about one of her friends being crazy, she smiled a smile unmistakable as friendly, I then asked her during the conversation about the traffic in the parking lot cause I just got a new car. She was interested enough to ask about my car and when I showed it too her, she said it was a very nice car (no sarcasm in her voice, I would have noticed it). The second I witnessed her fall over accidentally and I asked her about it and joked a bit, and that same smile appeare again.
I was so dissappointed in myself on Thursday when I tried to tell her I like her, the words formed in my throat, started coming out when they got clogged at th end of my mouth, I could not speak the words. I beat myself up so bad that I actually got sick, nearly collapsed I felt so ill. Until I realized it and I talked with my co-worker about it. He is in my grade and actually friends with this girl. He told me she held her nose high in the air, meaning she acted too good for people, adding "not to discourage you or anything." Which actually encouraged me since she had not hinted at being that way when I spoke to her. I keep asking myself, if she holds her nose in the air, why did she act so shy-like when I was talking to her? The answer was apparent to me but I still can't tell her for some reason.

Now to my need. I want to know what I am doing wrong or what I should do to speak "Hi (girl's name), I wanted to tell you that I really like you." I want to say it to her after school which is my prime time to ask her cause she and I park pretty close. I need advice on how to accomplish this since my method ony helps psych me up.

ask her out before the day comes and you'll be thinking what shoulda, coulda, woulda been :)

Just believe in yourself and what you have to say I guess. Don't judge her based on someone else's opinion and trust your gut instincts.

That someone else's opinion helped me actually because I know what he told me is true. But when I talked to her those two times she was completely different, she seemed shy. Not at all how she appears in the halls and classes. Which gives me the greatest boost to date. The next day I know I will have a shot is Monday after 3:15 (end of last class at school). I am more than sure I can do it, I will be looking for the tinyest thing I can use for the conversation starter. Luckily it is a "gold" day, meaning I have class with her third block so the chance of me finding that starter will be increased.

If I freeze up again, with time to spare, do you have any suggestions for what to do to unfreeze? I am the kind of person who likes to have a backup plan incase something goes wrong (I guess inherited from robotics).

Walking up to a girl that you only have spoken to twice and say "Hi, I like you" is generally not a very good Idea.
I have tried it out many times and never got what I wanted.

Try to talk to her a bit more first, get to know her better and don't tell her that you like her, show her... works much better.

There was one more conversation I had with her that was less than a minute because it was in the lunch line check out. I had gotten behind her (not intentionally) and noticed she had cash, I always pay cash so I asked "So you paying cash too?" She said "Today, yeah." with a slight giggle and a smile like I had told you before. This was the same day of the first conversation I told you about, it is the class we have together and we have first lunch (when the tardy bell rings we go to lunch, so before class).

Last time I 'showed' I liked someone (last year when my mind was clouded), I gave a girl a rose for v-day and it began to die after that day (yes I had hung around that girl to get to know her). You can see my hesitation of 'showing' I like her until I know she is okay with knowing I like her. Glad it ended, don't know what I saw in the other girl (she creeps me out now, always read manga) but I see everything in this girl. I have been getting to know this girl since school started. Its a shame I have waited so long to talk to her.

I believe signs are obvious but often missunderstood by the untrained eyes and ears. I have learned the trick is to pay attention to small things. 2 signs that have appeared within the last 24 hours are hearing the girl's name at the robotics competition at lunch time, of all the names in the world that is the one I hear. The second I just opened a random thread in this forum and the poster's name is the same as the girl's. I don't expect anyone to understand this but over the years I have been able to make accurate predictions based on this method.
On Thursday, I had a dream of telling her in the parking lot. When I woke from this dream, my sterio was playing radio and the song that was playing was hanging by a moment (my favorite) but it was at the point "There is nothing else lose, There is nothing else to find, There is nothing else in the world that can change my mind" Of all the points in the song, these are the most significant words in the song to me.

Omg this is so adorable! okay, you should try to become friends with her, if your friend is friends with her then he can contact OR maybe its better if you fly solo and do it urself if you think ur friend might be too.... controlling.
Keep talking to her.. but ask her out before someone else does. Girls like flirting, even if they say they dont. trust.