Dear …
__(Some time/A short while)__ ago, on __(date)__,  I had an interview scheduled with __(you/your organization)__ which I subsequently had cancelled because of an offer from my existing employer.  At the time, I was torn between that offer and following through with the interview with __(you/your organization)__.  It was a decision that was a very difficult one for me, but in the end I decided to take the offer from my current employer, a company that I was already familiar with. 
The new position they offered to me was quite appealing in the beginning.  However, having now worked in that position for __(# weeks/months)__, I have realized that this was not the best choice for me after all.  It has become evident that it is clearly not a good fit for me, hence, my letter to you today.  I am writing to you in hopes that you have kept my resume on file and that there may still be an employment opportunity for me within your organization.   
I am asking you to once again consider interviewing me for a position with __(name of company)__.  I can assure you that given a chance to prove my abilities, you would not be disappointed with my performance.  Any opportunity to discuss this further with you would be greatly appreciated.  For your convenience, I have enclosed a copy of my current resume. 
Thank you for your consideration of my request for an interview.  I ask you to kindly contact me anytime at __(phone #)__ and look forward to hearing from you soon. 
Sincerely,