When my thoughts trouble me deeply, depending on what they are, I just let them flow right through me without clinging onto them and dwelling.(A mindfullness thing) If that doesn't work, distraction is also a great tool. With relationships ending it is only natural to think things over a lot until you can come to a place of peace. It has been two and a half years since me and my ex-husband split and I still have days where I find myself dwelling on thoughts of him but I must say it happens less often and at a lower intensity. Hang in there the thought will lose their power in the end.