Subject: I'm sorry. Please read this.

I know this is inconsiderate for me to do by email, but I need to tell you something important, and it's hard for me to say in person.

I think we should see other people.

This is a life decision I needed to make for myself. Please don't take this upon yourself, wonder what you could have done differently, or think you're less of a person because of this.

You're truly a great person, and I've sincerely enjoyed the time we've spent together. 

I don't regret us being together at all. This is a decision I need to make, and I'm sorry if it hurts you. This is something I simply need to do.

I hope this break will be amicable for the both of us. If you're upset, I understand. I wish no ill will towards you, and I hope you find happiness.