Hi Rob,
Great to hear that you have been experimenting with meditation, and sorry to hear that you are finding it tough. It certainly is very simple in theory but frustratingly difficult in practice.
Might I be so bold as to suggest a slight change in emphasis. It is not really possible to do much about what the brain does, the key is to change your relationship with/reaction to its outputs.
You have already noticed that the brain churns out twaddle most of the time. This is a key insight that most people are not particularly aware of. The brain 'thinks' all the time and our default is to get carried along with this thinking as if we created it, so therefore it must be meaningful.
Your brain will carry on doing this whatever you do, but you can get much better at simply recognising it as thinking and get on with your day. It breaks the link between thought and mood/action.
Quite a good strategy is to have a really good think about the 5 or 6 things that you really value in life, the things that would make you into the person you want to be. Keep meditating and get better at recognising the nature of thoughts as they pop into your head. This will give you the necessary distance to be able to 'hold up' the thought against your values and choose to run with it or let it go.
Thoughts that don't get a hold of you very quickly fade to nothing. The more you ruminate on unproductive thoughts the more often they will appear. You can't stop thinking, but you can learn to accept whatever comes up and only act on the good bits.
People's natural assumption is that they are their thinking. This is not the case. In my view it is more accurate to say that you are the conciousness into which thoughts appear. This is a neutral, observing space into which all thoughts and perceptions make themselves known. It is the rarely visited space between thoughts. It is where Wes D and Bolam go when they are snowboarding.
Breaking the stranglehold of your thoughts (lessening their duration) trains your brain to spend more time in this marvelous space between thoughts. The clouds of your thinking drift on by and the world starts to look like a very different and much more interesting place.
Good luck with it all, and keep us posted on how you get on.