I've had tinnitus pretty bad for at least 15 years. Mine is comprised of 2 or more extremely high pitched frequencies, rather like the sound of escaping gas. I think it was basically noise damage, from the early 90's rave scene and extended listening to music on headphones at high volume.
I had a big crisis about it about 12 years ago, during which I was surprised to discover that most of my friends have it to some extent as well. I think we all screwed our ears a bit back then! It could also be related to drug and alcohol use - it's not just ear related but brain related as well. There are many different causes of tinnitus. For some people it's tension in the neck and jaw. For other's, it's the sound of blood wooshing around their head. Some people even have "objective tinnitus" which is actually a sound in the ears that other people can hear if they listen closely enough.
I think the thing to remember is that tinnitus is very common, and that your brain can learn to live with it. Despite mine being quite loud it doesn't really bother me much at all, unless I'm in total silence. For that reason I always have a fan or something on when I sleep.
There are preventative measures - alcohol and caffeine can make it worse. Smoking weed can make it seem worse although that might just be that it makes you more aware of it. Eating a healthy diet can help. As much natural food as you can. Make sure you're getting enough zinc and magnesium - most people are deficient and those minerals are important for ear health. Magnesium can protect your ears from noise damage and indeed the military has done research into using it on troops who have experienced loud noises in battles and in training.
The list of musicians who have tinnitus is huge. Noted sufferers include Chris Martin, Pete Townsend, Eric Clapton, Jeff Beck, Robert Plant, Barbara Streisand. You can get it from one noise related incident. For Townsend it was when Keith Moon blew his kit up on TV with him standing right next to it. For Andy Patridge of XTC it was when a careless studio engineer triggered a snare sample in one ear at full volume. For William Shatner it was a stage explosion when filming an episode of Star Trek.
You might want to look into notched frequency therapy. It works best for tinnitus of lower frequency because you have to be able to identify the frequency of your tinnitus accurately. It involves listening to white noise or music with that frequency notched out. Apparently this has been shown to greatly reduce the volume of tinnitus, if you do it for a few months.
Don't worry too much about it though. There's no reason why it should affect your life if you take the right frame of mind. And I'm pretty confident a cure is not far off. There have been some advances recently.