At 15, your chances of passing are excellent, and the ease to do so should be great, too. Of course, nothing is a guarantee of anything, but I think it's safe to say that you have roughly an arbitrary, very figurative 25-50% advantage over me, starting at 21, if you start now.
A few proven and unproven facts compilated by the non-expert me:
-People generally grow until they're 25. By starting now, you stop (or at least greatly slow) the process.
-Sometimes, a reversal is seen in masculine features developed before. The younger, the more likely they are to happen, and the more significant they should be. At 15, I think your chances are fairly good.
-The younger you are, the more the female characteristics will develop, statistically. And the younger you are, until a (pessimistic)18~(optimistic)25-year barrier, the more characteristics you will probably develop. For instance, it's believed that very few people starting after those years will develop female hips to a satisfying level, if anything even happens. Also, data seems to suggest that it's unlikely for breasts to ever reach Tanner stage 5 if transition is started older than that... even though that data can also be explained by the controversed use of progesterone and/or sub-optimal hormone regimens.
-In the vast majority of case, even if beard is present at 15, it won't be at its maximum for another 3-8 years. Starting now equals less hassle about
-I don't know about others, but my Adam's apple appeared towards the end of my 20th year. That's a little fortune you could save in tracheal shave.
-During the same year, my voice was partly ruined. I want my voice from a year ago back. If you're like me, trust me, even though a ten-year-old's voice is definitely preferable to a fifteen-year-old's, at twenty, you're gonna cry.
-Transitioning now means you can just change high schools if desired (fairly simple), live the hassles of social transition more easily and probably with less suspicion (a clueless 16-year-old is late, but a clueless 22-year-old definitely has something wrong somewhere), and start college/university/whatever as a a somewhat "full-fledged" female, without having to transition on the job or find another school with the same program.
To sum it up, even though starting in the early stages of/before puberty is the best, you're not far behind by starting during it, at 15.
Edit: I'll disobey you and still tell you that professional care is a lot better. But if for whatever reason you must self-medicate, BE CAREFUL. I'm going to give you these basic safety tips, which I hope will not be removed by moderators: I want it to be clear that I'm not encouraging you to self-medicate, just making sure that if you do, you put yourself at the least risk possible. After all, not only are you self-medicating, but you're doing it at at time of your life when your knowledge and wisdom are low AND your body is a complicated thing to manage.
-Have your levels checked (if your country allows patients to request tests themselves)
-Have risk factors checked, such as possible liver problems and family history of thrombosis, etc. checked. If you can't get them "honestly" checked, check for preliminary symptoms of those troubles and pretend you have them, so your doctor makes you do blood tests.
-Be extremely cautious with your doses, and never increase them quickly. More hormones than the maximum recommended increases risk, but REDUCES results, even at long-term.
-Use documents intended for professionals, read them carefully and make sure you understand them well. This might need research, since doctors obviously understand those things much more easily than you and I... Things like this document:
http://www.bgtransgender.com/Guidelines/guidelines-transgnder%20primcare.pdf-Don't go for injectables. Those are hard to do, arguably (yet also very contestably, as some say the exact contrary) less effective, but most importantly, they require to either be administered by a professional or someone who was carefully shown by a professional how to do it. Pills are most probably going to be the safest, simplest way for you to do it.
-Make sure wherever you get your medications from is a trusted place that will give you precisely what's on the label.
-Try to get unofficial help from a professional who knows what they're doing, if you can't get official help.
-Meet a doctor if complications arise. If needed, find a lie to cover up, but don't keep yourself from a possibly life-saving treatment out of fear or shame.
-Get an anti-androgen. Just estrogen, combined with high levels of testosterone in your teenager's body, might produce unpredictable results, such as more testosterone overall (conversion from estrogen and/or reactive additional T production), potentially toxic levels of hormones without effects, and probably a lot of other things.
