Its not among the easy manifestations.
For me, if I were to do it...
First and foremost, you need to focus on yourself and have absolute certainty in yourself. The more love you have of yourself, the more powerful your energies will be. The belief that "she barely remembers [you] exists" needs to go - you must believe yourself to be unforgettable, absolutely powerful, complete desireable, and that she /is/ already kicking herself for not coming for you. Having a vast confidence in your ability to manifest on every level also helps.
Next, you should consider that 'she' does not exist. There are many ways to consider this, but 'she' does not have any mind of her own because in subjective reality, she is only part of yourself. If she is rejecting you, it is because YOU are rejecting you. Why are you rejecting you? Why aren't you feverishly in love with yourself? You manifested her into her universe, and you manifested her lack of interest in you. Why?
Once you've visualized a new her that is vastly interested in you and remembers you, watch for alpha reflections. The universe will offer you opportunities - pay attention to them. Its likely that one of them will bring her to you.
Finally, try not to think too much about it once you've done a few manifestations of her. Attachment to result will hurt your manifestation. And it will take time, regardless. Focus elsewhere.
To be frank, it will probably be easier just to manifest an ideal girlfriend rather than a specific girl - and probably happier for you. But if you want to, it is possible.