How do you leave someone that you have been in a relationship with for 20 months?

Recently, i've noticed that its been getting hard to say "I love you too". Thats when i new something was wrong, now seeing her is becoming a bit of a chore. I havn't been spiteful or anything like that, i just can't help but avoid any kind of girlfriend-boyfriend chats. How do i end something this long? I really don't know :(