Simply replace the unfilled areas such as "" and "[date]" with your information.
As our email exchanges have been growing in intensity, I've have had to take some time to examine my expectations for our relationship. When I look back over the great things we have shared online, I feel very fortunate to have met you. However, I also realize that you want more out of this than I am willing to give.
You are a [positive adjective] and [positive adjective] person. From my emails, you may have realized that I simply am not as [adjective] as you are. Therefore, I must end this before you invest too much in me. You are a wonderful person and any [guy/girl] would be lucky to have you. Please believe me that there is someone out there waiting who is better for you than I am.