Well, I can tell you my own experiences, but they aren't necessarily going to mirror yours.
I spent a long long time trying to tell myself that I was both and could be both. A really long time. During this time, I became progressively more and more depressed and fed up with existence because I was not able to be female at all outside of online. After one of my best friends who was also trans overcame a ton of stuff and finally began her transition, I sat there and realized that I needed to do something before any more time passed. I realized the only reason I wanted to be male at all was so I didn't have to tell anyone I was trans, and so that I didn't have to "settle" for a transitioned life. I wanted to literally create a brand new female identity that had never been male - it was an impossible goal, and keeping it impossible was my coping mechanism for not transitioning.
So I came out to my wife, and started making active attempts at beginning my own transition. I questioned my course heavily for many months. I've now been full time since February, and I STILL question this every once in a while. I don't think that ever completely totally goes away. BUT it no longer freaks me out like it once did.
The two things I did and continue to do that helped me deal with those nagging feelings were to think back and mentally account for all the things that told me I needed to be feminine during my life. And a series of thought experiments.
For the first, I would sit for long stretches of time and just ruminate on my life up to that point, on my childhood, on my teenage years... what had happened, why it happened, what the signs were. It's taken me a LONG time to piece my life back together, because I ripped those memories up when I was still in coping mode. But piecing them back together forced me to re-analyze very closely the course that had taken me to where I was, and almost every step of that path told me I was finally turned the right way.
For the second, I devised multiple scenarios of a generic type - public events, private events, dates, hanging out at home, just about anything - life stuff. And I would mentally create a purely male and purely female avatar for myself, and imagine myself doing whatever that was in the guise of both genders. Neither of these were ME - they were just unidentified generic gendered people so my own identity hangups did not come into play. I would commit myself to my imaginary role as much as was possible, and examine my reactions to existing in that form. My reactions told me I needed to be a girl.
So now when I doubt myself, I think back to these mental exercises and their results, and I usually am able to talk some sense into myself.
Now, here's a thing though. I spent 27.5 years being raised and treated and living as an american male. I will always act with mannerisms and habits that reflect that, no matter how female I might be. I will occasionally have thoughts and instincts born from that quarter of a century of gendered experience and react to them in a traditionally masculine way. And that's OKAY, and it doesn't make me any less of a woman. There is no law that says I have to act a certain way to be a girl. I like cars, I don't pretend I don't know how they work, I take charge of situations when it's necessary to do so, I speak up when I need to be heard. I also giggle and laugh and love when my wife takes my hand walking down the street and fail to understand most ball related sports. These are societal constructs of masculinity and femininity. I have some of both - probably more masculine traits than I would've had if I had been born with the right body, but that's okay.
And it took me a LOT of effort to think this way. To be okay with the fact that I have those experiences and they will always affect my thinking.
So, I just wrote a small novel on the topic. This proves that yes, it's normal to think about these things and no, you are not alone in having doubts. But your personality doesn't need to be such an effort - you are not obligated to act girly to be trans or to be female. Just be you.
