Hmm i think what you need is just some self-confidence, trust me, we ALL have the same issue sometimes, we lack confidence in ourselves and constantly blast our minds with "what ifs" or "what about"s the best way is to JUST DO IT! <copyright nike>

joking aside, just get her alone at any time ( you said you see each other at the hall right? ok thats perfect ) you can say something in addition to your nomal "hi!" try something like " hi <girls name here> !" and she will probably hi back or nod at you or smile, then before she walks off just summon the courage and DO IT just say something like " hey i know your busy but can i have a moment of your time?" and then just spring out a quick invitation on her, just be casual and normal about it dont act scared! " we've known each other for a while now and i would like to see if you could come after work today for a cup o coffee, if thats ok with you? " or something like that! look, u need to know if she feels for u the same way you do to her, so its best to find out, then rather than be all scared wondering " WHAT IF "

hope my advice helps, its just what i would do in that situation.