I often hear the same thing from other bisexual people who marry or date people of differing genders, who feel 'left out' online or had a bad experience they wanted to share on something like r/bisexual. But know that you are always part of the community. I believe that IRL experiences can differ from (largely negative) online ones. Sometimes, these online spaces become largely focused on adversity or bad experiences. I found my mental health improved when I avoided subs dedicated to my sexual orientation just because stuff IRL has been much better overall (eg, I've never had mean comments IRL about me doing my nails, quite the contrary, actually!)
Find good friends through hobbies and social events. I've found my strongest friendships when I have diverse friend groups spanning the gender and sexual orientation spectrum. Even straight people! :)