Step-by-step explanation:
Dear (name),
What is up with you lately? You've been going out so much and I feel that you don't care about both of us now... If it's like this, we're over and I'm sorry. I can't take it anymore since you're acting we're like far distance friends and not close. You have been breaking my heart so I think it's time for us to split.
Hope this is good (?) :))