I'm a Bisexual teenager & Have a hard time dealing with it. HELP!?

I've known i was Bisexual my entire life. The only person who knows is my sister, whos really supportive. Last year I tried to tell my mum who dismissed it as 'a phase' and also told me she doesn't believe in Bisexuals which really upset me. I've also been in love with someone my own gender for over a year and I know she feels the same because she kissed me. I need help in all those areas and it looks like the internet is the only thing willing to help me right now! I'll be thankful for all the answers i can get, thank you x