Her last thoughts before she fell asleep at night were of him, and her first thoughts when she woke up in the morning were of him. No matter what she was doing he was somehow related to it. She could be yelling at subordinates and he would just pop up in her head. Never in her entire life was she so infatuated with someone that her life basically revolved around him.
Yet she knew he could never feel the same about her. Lilah was just a release to him, and maybe even a way to get information on Wolfram & Hart. The sad thing is that no matter how much it hurt her inside that he didn't feel the same and was using her, the fact was it was her that he went to when he was frustrated or hurting from what happened with Angel. Sometimes right at his climax he would murmur Fred's name or in the middle of the night when she would watch him sleep it would be Fred's name whispered from his lips with a ghost of a smile.
That wasn't the worst part though, the fact that he never really said her name the way he should hurt her the most. She would dream about him actually looking at her and saying her name the way a lover would and a smile would pass over her lips. No one would ever really expect her to love anyone let alone Wesley, but anyone that took the chance to really know her and get past all the walls she put up could clearly see it in her eyes.
The problem was no one took that chance to know her, not even him. In her last moments alive it was the thoughts of him actually loving her that put her mind at ease. Although in the back of her mind she knew it could never be, he loved another and would always love another. It could never be her no matter how hard she tried and when she died she accepted that.