Hi Helena,
Welcome to the big board of advice! So many perspectives and lots to share in this hall of mirrors. I am happy to be the 'man dressed as a woman' figure in your story, but I can still hear the precision with which you are describing what you feel, and your plans. Once we decide to escape our gender prisons, we never turn back, because the underlying motivation is very simple- we want to feel more of what life has to offer.
The importance of this is that you might as well tell your girlfriend now and save her the grief of spending time with you if she turns out not to feel comfortable with you 'pretending to be a woman', or more generously stated- 'enjoying as much as I can of the woman's world'. You want to pair with someone who embraces that, and the sooner you start showing yourself the sooner you will find an SO who does!
