Sorry I have been quiet - I was away for a couple of days.
Your description is excellent, very clear and it seems like you have a good understanding of what is going on. But what to do about it?
As you say, you seem to have a tendency to 'dream', which is a useful ability in itself, but more useful when you can use it when you want to, not all the time!
So how can you maintain concentration when you know it is the best thing for you to do?
Well basically, you have to give your brain the message that what you want to focus on is highly important. And how do you do that?
Well, put simply, you need to arouse some sort of mild emotion that compels you to continue paying attention.
So, some examples:
1) You are listening to someone and your attention starts to wander. You could get a little irritated with yourself and make yourself concentrate out of politeness. Or you could deliberately become fascinated with what the person has to say - ask questions, make agreeing noises - anything that keeps you involved and gives your brain the message that "this is important"
2) You are reading a paragraph that is tricky and your mind wanders. You could try reminding yourself that self discipline is a highly valuable ability and you have to practise it at every opportunity. Go back to the start of the paragraph and start again. If your mind wanders, start again and so on until you get the whole paragraph. If you give yourself no choice but to finish it, in time you will do it first time.
I think sometimes we need to treat parts of ourselves like children who have to be shown the right way to do something. If a kid understands that the only way he can get our to play is to do his homework, he'll do that homework much quicker. Once your brain gets the message that every time it wanders it has to start from the beginning again, it will soon shape up!
3) You are studying and you are losing concentration. You could try motivating yourself by thinking how bad it would be to fail the exam. Or how good it would be to pass well. Forcing yourself to think of consequences is a good way to motivate yourself. Ignoring them is a good way to let yourself 'off the hook'.
I hope this helps. As with all things, gentle, persistent, unrelenting effort.