Resignation was a Mistake - Requesting Rehire
Recently, I was offered a position with __(Name of Company)__. Though the decision was a difficult one for me, the new challenges as presented seemed rather appealing to me at the time. Therefore, I decided to take the offer from __(Name of Company)__ and resigned my position of __(name of position)__ with __(Name of Organization)__.
I have now come to realize that this was not the best move for me after all. Having worked for __(Name of Company)__ now for __(length of time)__, it has become exceedingly apparent that this was not at all a good fit for me. Hence my letter to you today; I am writing to ask you to consider hiring me back for my old position of __(name of position)__.
Clearly, I made a error in judgment when I thought that working for __(Name of Company)__ would be a beneficial career move for me. I do regret that decision now. But if you are willing to overlook my mistake and hire me back as a __(name of position)__ with __(Name of Organization)__, you can be well-assured that you will receive 100% effort from me.
An opportunity to discuss this further with you would be greatly appreciated and I ask you to contact me at __(phone #)__ __(at anytime/in the evenings/etc.)__. Thank you for considering my request. I will remain hopeful and look forward to hearing from you soon.