I have a couple answers to this that might be uncommon, personal, and wouldn't have helped me in the early stages, but were the final nails in the coffin of this doubt for me and I haven't ever worried about it since.

The first came a couple months after coming out. I noticed that I had already changed a lot, almost entirely mental. I couldn't describe exactly how, but it felt like I really had done myself a favor and burned the bridges I needed to in order to take control of my life. At that point, I started to figure - well, if this whole being-a-girl thing doesn't work out, who's to say I can't transition again? I couldn't imagine going back to who I was before - I knew that if I was going to ever identify as masculine again, it'd be a retransition, not a detransition. And tbh if that ever happens I very much look forward to what new roads lie in front of me. It's nothing to be afraid of - everyone I fell out with in the process of coming out was no real friend of mine anyway. And I know the people in my life now would have my back.

The second was that I developed pretty severe fibromyalgia after some time on HRT. I think I had it to a low grade before? But it definitely worsened to a disabling degree after about a year on hormones. It's not a very well understood condition (and as a diagnosis of exclusion it's probably not just one condition) but it's a lot more common in women, which maybe implies it's just part of how my body works on estrogen. So I had a choice to make - would I rather go off estrogen if it'd help with the pain? And the answer was a surprisingly immediate and definitive "hell fucking no". Even with a new disability life was so much better. That's the point I knew it was the right choice and I've never doubted it since.

I guess the way I'd tie this up is - it took a long while after I started giving it a go to be 100% assured I'd made the right decision. It is a leap of faith you will have to make without a guarantee - that said, if you're thinking about it to this level your odds are probably extremely high. And you'll know pretty quick if things like HRT are for you or not.

You might also benefit from nonbinary identity in the meantime to give yourself the space to explore any and all options. I landed on identifying as nonbinary but broadly transfemme - you can figure out the more specific parts of your identity later, just figure out what you want to explore in the present and you'll get there with some time!