As a chronic overthinker myself, all I can tell you is that thinking rarely accomplishes anything productive unless it is combined with doing. Thinking about standing up is pointless unless you actually do stand up, for example.
Thinking about undertaking a massive shift in your life is an important thing, you can't just rush into it unprepared. But there comes a point where the enormity of the task at hand can also overwhelm the mind and cause it to shut down, become depressed or crawl under the blankets and refuse to come out.
If your concern is transition I'd suggest you break it down, don't contemplate the enormity of it just the parts you need to engage with, and can handle, now. I'm not sure where you are now in the scheme of things (sorry, should have read your sig before I hit reply) but if it's at the very start I'd suggest just trying to find a good gender counsellor and make an appointment, that way you have one task nailed down - they should be able to help you start to plan the way ahead. If you already have one and they're not helping, maybe it's time to find one that can. You shouldn't have to do any of this alone - hopefully there are good professionals out there that can help support and steer you in the right direction when you feel you need advice and help. Failing that there's always this forum!
Just don't overthink the situation and you'll find you can cope with the situation better because it won't be so overwhelming.
