If you are not ready and she is forcing you, then yes. Leave her.

How bout you tell her you're sleeping with someone else, then she'll really see how much you love her...
If you loved her you wouldn't be sleeping with someone else.

I know what you are going through. My grandmother and I were very close... It's been six years and sometimes I still cry...though not as often. The only thing that will heal you is time... In the meantime think of all the good times you shared with him:-)