My suggestion is have a good talk and say Good bye. there is no point lingering in a relationship where you know you have no future. Because if you keep giving her time, you will never be able to break up and the pattern continues and you get used to it.

I was in a relationship like that and it was so hard for me to break up with the guy even when i knew we had no future so i had to make a very firm decision and let go of everything.

Change your phone and do not keep in touch after the breakup at least for few months. Also, it is going to be very painful so make sure you have a good support of friends around to visit, talk to so that you can divert your mind, etc. Think of this as an addiction and how difficult it is to get rid of it. I used to think it would be easy but it took me more than a year to be normal again so do not expect a miracle if you have really loved her. Do not date someone else immediately also as you might not know if that is the right person and you could be only rebounding in the process and thus getting hurt again.

All the best!