How can I know for sure that this is who I am, and not just a "phase" like some people have said?

Unfortunately, nothing is for sure because you're always changing as you grow as you learn more through discovery. But that doesn't mean you can't be confident in who you are now, should it change or not. In most cases, you'll know when you agree and/or identify with a certain label. It's like your soul finally find out what is missing, and there you have that click. The second possible factor would also be to identify with the label for a long period of time for perhaps a few months and so on. It will come to a point where it sticks and you don't find yourself identifying as anything else.

Sexual experimentation is perfectly normal, only you will know you - if you are LGBTQ+ great, if not that is also fine!

You may not be able to know that right now, but after time, you will. Remember that you are supported by life, and that everything that needs to be revealed to you will be at the exact time you need to know about it. Whether it is a phase or not, you are okay just the way you are. :)

There's no such thing as a phase, what you can do right now is just acknowledge your feelings and talk about them with someone who supports you. Time will help figure rest out.

Getting to know about yourself isn't an overnight process. It takes a lot of time, effort and experiences. We learn from our mistakes. We realize our potentials as well as weakness. However, it is ultimately we ourselves who can understand us better than anyone else. Never come to conclusions about yourself by looking at periodic feelings. They might be temporary and wouldn't take you to the actual answer. Look for long time gut feelings and sum up. Life is a long journey of these and trials. We may not be the same person as time goes. Yet it's important to have a perspective of your own.