I have this problem too and I can definitely tell when the thoughts start creeping in. My art really tenses up. I wish I could just turn my stupid brain off!
I usually have some kind of music on and it's been helping. At first, I only listened to classical music so I wouldn't focus on the words but it was too easy to tune out. Now I have a mixture of music constantly playing - rock, jazz, soundtracks, anything and everything I can get my hands on. I've noticed that headphones work better than a stereo. Headphones get the music right in my head and sometimes, if the thoughts won't quit, I turn the music up until I can feel it.
I also take a break as soon as I feel tense or rushed and work on something loose, like doodling. I've also noticed that sometimes, when I'm working and my brain wanders, I need a harder subject, something that makes me constantly check the reference picture and I literally have to talk to myself, asking, "Is this shape right? More curve? More shading? More texture? Less highlight?" That gets my brain working on something other than the crud of life or whatever.
Hope that helps some. Keep your art as your safe haven and defend it with all that you've got. It's very precious.
There's a saying from writer Dorothy Parker: “I hate writing, I love having written.” I feel the same way about drawing sometimes. You might be interested in this journal entry I wrote which offers tips for artists: [link]