Bladder problems? Please help! It is ruining my life :(

I pee way too often.
Sometimes, I get this strange feeling that I cannot push out all my pee, and have a strange sensation down there that I really need to go. After about five minutes, my bladder will have more urine in it and I have to go to the toilet again, only to have the same problem of not being able to finish my business.
This has become much more frequent, and has begun to occur every night. Because of this, I am not getting enough sleep and it has been affecting my school work and performance.

However, I went to the GP and took a full blood test. The test came back clear, and there was no sign of any infections or diabetes.
I am a fourteen year old girl, and this has been occurring since I was about six. However, during the last month or so, this problem has become stronger. Usually, I would get it about two nights during the week, but this is happening EVERY SINGLE NIGHT, and often during the day too.
I had to miss a school trip last month, in fear that I would suddenly start going through one of these phases when there is no toilet around.

When I am not going through a phase like this, I usually go about every two hours - which is still more often than normal. However, this could simply be because I have a weak bladder - which is easy to train up (as I have heard) by doing pelvic exercises. However, this does not bother me that much.

In my last question, a young boy with the same problem as me said it was something to do with a muscle connected to the bladder which does not let all the urine pass out. It can contract randomly, therefore not letting the bladder empty fully.

I am in complete despair! I don't know what to do!
This problem is actually ruining my whole life, as trivial as it sounds.
I cannot go on many trips because of this, and school is becoming a problem as I sometimes go through these phases during lesson time.

It really isn't affected by the amount of water I drink. I could drink alot of water but still not go through a phase, or I can not drink any water for ages and will go through this phase or vice versa.

Please please please help. I really don't know what to do anymore!
If this was an infection or something, it would be easier to deal with.

Also, when I start going through this phase I get a strange sensation that I really need to go, but it's much closer to my pee hole then when my bladder is actually full?
It feels really irritated, but at the same time my bladder manages to fill very quickly.
However, as I said before - when I do go, little pee comes out and it is very clear.
Last night I spent about twenty minutes trying to push it all out in order to go to sleep, but failed, so eventually I had to go and hold it in.