Newbie Q's

I first tried Ashtanga Yoga over ten years ago but only stuck with it for a couple of months as I didn’t really like it. I started again around six weeks ago and really enjoy it this time around but I seem unable to practise without all sorts of day to day stuff running through my head, how can I clear my mind? I also run daily and train Muay Thai ( my first love ) what do you more experienced yogis think of this combination? Thanks:)