Why is this woman affecting my emotional well being so much?

In my office there is another lady who enjoys a reputation for being attractive and likable. From the bottom of my heart, I do not think she qualifies in either category. I have always seen her to be very self centered and self serving, with a enormous conceit. Women really like her too and enjoy talking with her. She has always looked at me as if I was not attractive, even though I have always been considered to be. Once she made sure that I heard that she was going to get the skimpiest bikini and go to a beach where there were a lot of guys. I pretended not to hear her, but she looked over at me in a contemptuous way, as if to say "you couldn't do it". This woman has gotten on my nerves big time. She is about 50 years old and acts like she is 20. She is extremely small in stature, under 5 feet, and weighs under 100 pounds. I am 5 feet 4, and am overweight at 150 pounds, but still look good. In the last month, I have gone on a diet and purchased a lot of cute clothes. This is all due to this woman. She acts like she has it all over me. Men go gaga over her, and women (even very unattractive and very overweight ones) just love her. They don't feel threatened by her like I do. If you could please tell me something comforting to make me feel better about this situation, I would greatly appreciate it. She makes me feel like I want to quit my job. Why doesn't she try to make the ladies who weigh over 200 pounds feel inadequate? Why me? Please say something to me to make me feel a little better. Thank you.