How to tell him I don't love him anymore?

How do you tell the man you've been with for three years and have a child with you don't love him anymore? I've been having a lot of what if thoughts in regards to what life would be like without him and I can easily picture a future without him. We argue often be have always made up. We have sex and it's fine but afterwards I find myself just wanting to be alone. I'm not sure why I feel this way.