It helps me to remember that I grew up with a lot of really negative messages and images about transpeople, almost like marinating in toxic ideas. So of course some of them soaked in. But it helps when I have them to call them out in my head as thoughts I inherited, not ones I came up with or want to believe.
So, as often as I can, when I notice myself having one of those thoughts, I go "wow, look at that transphobic garbage kicking around my head." I still have the feelings, but I spin in them less when I challenge those beliefs.
Also, I remind myself, just because you have a certain thought, it doesn't mean it's true.
Plus, just know that you're not alone in doubting yourself. If I had a nickel for every trans person I know who struggled with not being "(fill in the blank) enough," I would seriously be rich. It's crap, and it's toxic. Don't believe it. Keep fighting it. Find people who celebrate you and support you (like in these forums). Hang in there!