Just a thought is all it is. You can change it, deny it, believe it, watch it, follow it, ignore it or just let it be. You can do any of these in theory. In practice the choices are often less clear. Those thoughts that take hold of you, boil your blood, make you perspire, raise your heartbeat and wreck your sleep are so resistant to influence. They have you in their grip and will not leave you alone until their work is done. You know they are 'just thoughts' but seem so much more as they play with your mood and direct what you do. They present the problem to the machinery of your mind which will then grind out answers seeking the one to appease the thought riding you like a wild stallion under its control. Break free and throw it off. It can only rule you if you let it. Tame it yourself as you pick it up, write it down, look at it and call it for what it is. In the same way that now is all there ever is a thought is always 'just a thought'.
ps...I wrote this for myself this morning as I noticed some thoughts taking destructive control of me.