First of all, wipe that "I'm so ashamed" attitude off the table, because it's pointless and... well... a bit hurtful.

It's perfectly fine and valid that your identity changes or moves or shifts. People find their sexual attractions change at all stages in life and it's really cool.

May I ask why you're so content on not being not straight? What is it about straightness that you're so uncomfortable and afraid of leaving? What is it about gayness/biness/panness/whatever identity word you may find that you're so uncomfortable and afraid of entering? Usually when a straight person expresses fear of becoming gay, it's because they have a lot of pent up homophobia that they haven't dealt with. This person may feel to be a huge ally to the MOGII community, but they end up having mounds of homophobic feelings to work through.

At the end of the day, hopefully you'll find that there's nothing to be afraid of and straightness isn't in any way better than anything else. You'll embrace yourself, your experiences, and your feelings. You'll experiment and find words for yourself. You'll find pride.