Constant Lightheadedness: What Could Be the Cause?

I've been getting lightheaded a lot recently. Like whenever I stand up, or turn my head, or just move the wrong way, I get super lightheaded, I usually have to grab onto something around me so I don't fall, my vision goes completely black for a few seconds, and I hear a really loud ringing in my ear that blocks all the other sounds around me out. I have no idea what's causing this, and it seems like nobody I tell is taking it seriously. Even doctors just brush it off before I can even say much about it, but I really don't feel like it's normal. I've tried everything that would be the easy answers, dehydration, iron deficiency, maybe not enough food or protein, but it happens all the time anyways. This is relatively new, maybe happening the last year or so, but I'm concerned something is actually wrong with me, or it's going to get worse and I'll start to actually pass out.