Your mum is of a different generation and so coming to terms with bisexuality and people being gay, crossdressing etc is something completely alien to her.
Your mums generation would have put people in jail for homosexual acts and crossdressing etc would have been viewed as gross perversity. It was something that was to be shunned and ignored and punished if it came out into the open. Homosexuality wasn't leagal in UK until very recent times.
You need to tell your mum that even though you are bisexual, you are not suffering from a terminal infectious disease and that your feelings for her havn't change, neither will they ever.
Tell her that it is no failing on her part, OR YOURS, that you are the way you are. Tell her this is century 21 and that bisexuality has been around for millenia, but only now is it allowed to come out into the open and IS TOTALLY acceptable in todays society.
Many film stars were gay/ bisexual, but they had to hide it away. It didn't stop them from being loved by the public and didn't stop them from being famous. So many young men wanted to be like Rock Hudson or Dirk Bogarde, yet both were gay.
Tell your mum, you love her very much and that you are grateful to her for bringing you into the world and that you wouldn;t change a single thing. Tell you are happy with who you are and that she should be too. X