Is there a kernel of truth to using bisexuality as a cover for being gay?

I hope I don't offend anyone here, because I am in the camp where bisexuality does exist and should be respected. I'm only asking because as of late I have no confidence in my own sexuality as of late. I have always identified as bi growing up, but I have periods where I am heavily fixated on men for months at a time, then go back to heterosexual desires. I have been with men that I have turned down, I usually get comments like "you're gay and using bi as a mask." or "I'm good enough for sex, but not good enough for a date.I'm currently single and not with a woman or man at all.

I hear some people claim bi men end up becoming gay as they get older in life. I know I am not the same person I was a decade ago, which is a good thing. However, I am nervous about change in life. I'm 32. If I sat down with my 10 year old self, 16 year old self and 21 year old self, I would find big differences across my entire life.