It took me a couple of years to be able to shut everything out of my mind and achieve visulisations...now I drift off at really strange points, lol...
I found when i fisrt started out (and even now at times) that focusing on a candles flame really helped me - and having a nice hot bath and doing it in there too...
Other people may have more ideas for you

Just a thought - have you tried using soft music?? Sometimes focusing on your breathing and listening to the music can drown out your thoughts...
xxx