Rejection hitting harder

There is a girl who works in the same building that the company I work for is located. She works for a different company though, I asked her out and she said yes. I was actually surprised as I really didnt expect her to, we exchanged numbers and I felt like I was on top of the world.
Later that night she texted me saying she had thought more about it and that she is not up for dating anybody at the moment as she is working on herself and her own mental health. I respect that. I understand that she is probably not attracted to me and/or doesn't want to risk getting involved with someone who works in the same place as her, as that could make things really awkward, especially if things don't work out well. I totally understand she has the right to change her mind and that she does not owe me anything, but damn, the disappointment really hit me harder than I ever would have expected.

I suppose it's because I havent really 'liked' someone in a long time now, and when she initially said yes, I got my hopes up only to have them crushed. I don't understand how this has caused me to feel so depressed, I am used to rejection, I have experienced it often! But for some reason I cant get her out of my head and I feel a huge sense of depression knowing that each day at work she is somewhere in the building but I wont be talking to her, as I absolutely do not want to pester her. A no is a no, I get it. I respect it. But this particular 'no' has hit me harder than many others I have experienced over my life.

Oh well, this is my problem and I will work through it as best as I can.