You don't need to stress about it, sexuality and all those sorts of things are changing with time, don't get me wrong, you may not change but if you like girls then you like girls too, its no big deal, love is love no matter who it is or who its for, if the people or environment around you is the issue with being Bi then surround yourself with people that are like minded and open, there are so many people that will listen and understand because they have been in the same space and situation but don't worry about it so much, in the end, you'll love who you love and it will be right for you, boy or girl :)
I am transgender myself and i once thought that i was bisexual, gay, genderfluid, angender. I found that i rushed myself into finding a label because i felt that if i didnt have one, i wouldnt be accepted. Now, i am out as a trans boy, going on hormones, with a lovely partner. I still dont know how i identify sexuality wise, i just find that when i like someone, i like them. Sometimes people do find comfort in labels, so if you feel that you identify with bi for a while, then identify with lesbian later on in life, that doesnt mean that the part of your life when you identified as bi is any less valid than the present. Phases happen, but that doesnt make them any less valid. You do you. If the people around you are good people, they will stick with you, and if they dont, theyre not worth it!