What if it’s a phase?
What if it is? That doesn’t stop you being asexual right now.
It may be tempting to hold back on accepting your asexuality in the hope that eventually you’ll ‘bloom’ into a sexual person. I’m not saying that might not eventually happen, but consider this: do you want to spend your life thinking of yourself as an undeveloped person, living for the dreamed of day when you’ll become whole? Might you feel more comfortable accepting who you are now as a whole complete valid person? Maybe one day you will “bloom”, and if and when you do, you won’t have lost anything by being comfortable in the mean time.
There’s no shame in identifying as one thing and then later identifying as another. Your identity isn’t meant to limit you. If you’ve moved on or changed, then by all means describe yourself differently. If you fear you might be different in the future, that doesn’t change which label is most useful to you in the present. There’s nothing wrong with change.