Hey guy. I really feel for you and want you to know that you WILL feel better, this will pass, etc. However, it's no fun being where you are right now - that I KNOW! And I'm worried about you. Being 15 and feeling like nobody gets you is a downright lonely, shitty place to be. Keep trying (like you've done by posting here).
If the notion doesn't turn you off, I'd suggest finding an older mentor-like someone to talk to. (I'm guessing your parents aren't much support. Even if they were/are, non-parent support is often needed as well.) If professional counseling is not doable right now (and it can be really a crap shoot to find someone that gets you, even there), even a guidance counselor, teacher, coach, or some other older person who has his sh!t together a little and gets you helps. I'm glad that at least you HAVE an older brother who has friends that sort of get you. Some people don't even have that.
Also - you might want to experiment with different artistic outlets (drawing, writing, music, film, dance, theater, etc.). You don't have to be "good" at it - it's purely for self-exploration and healing (and anyway - "good" and "bad" aren't what Art is about).
Also - don't discount the basics like proper sleep, a little exercise, nourishing food - it really does effect how you feel. And you may even be an appropriate candidate for medication - not necessarily pushing it, just saying from experience. My daughter - now 19 - sounds similar to you in some ways. She also had extreme physical symptoms which didn't seem to have a physical cause (after many, many tests). Finally, her doctor suggested something like Zoloft - and it actually made a big difference - physically anyway. She no longer was plagued by anxiety & fears that caused her to be practically housebound and unable to sleep. The help it gives/gave regarding her depression, while not as noticable, is still substantial. The medication is definitely NOT a HAPPY PILL - at least not for her or for me (I take it too). But it takes the edge off my depression enough for it not to interfere so much with my life. It makes keeping going easier to do - even though I don't always want to. So - for ME it helps a little...but a HAPPY PILL would be nice.
You've got a bright mind and a deep soul. Please keep the faith. We need people like you and my daughter - sensitive, compassionate, reflective types - to enrich our world, make this planet habitable and humane. It does take time and work to understand how to make your gifts work FOR you - not against you. PLEASE hang in!