Thank you for writing to AlterHeros with your question. I understand that you have two questions. Firstly, you want to know if your attraction to women (and one woman in particular) is a phase. Secondly, I think you are asking if this phase will pass or if it is worth starting a relationship with a woman.
Sexuality isn’t always a solid thing. Some people consider themselves queer their entire lives no matter who they are sleeping with. Others consider themselves straight as long as they don’t have relationship with someone of the same gender. What is important is to do what feels right, right now. Sometimes people come into our lives that can change things temporarily and sometimes they change them forever. You don’t need to fly the bisexual flag in order to want to date a woman. Deciding to move from lovers to girlfriends is a hard decision no matter what your sexual orientation.
Some questions you can ask yourself are: are you attracted mentally and emotionally to her, or just physically? Would you feel comfortable if she dated someone else or do you see yourself in that role? How important is she to you? You also need to ask some questions to other people involved, your boyfriend, and your lover. Does your female lover feel the same way? How does your boyfriend feel about you dating other people? Do you still love your boyfriend? If you choose to start a relationship with her, would you like to stay with your boyfriend too? Try to think about several aspects of this situation.
It might help to talk this through with someone outside of your relationships, whether that is a friend, relative, or a counsellor.
Hope this helps!