Lost in My Own Needs

I'm an artist and I've been in an open relationship for a year now. Tonight was the first time I went to bed alone after my partner left to see someone else, which is fine by both of us. Lying here, staring at the ceiling, feeling so empty, it's like the weight of loneliness has turned into this unbearable craving. My hand found its way under the covers, curling around myself, fingers brushing over slick skin and aching need. Fisting my hand hard, I imagine his mouth on me instead, pushing against every touch to get deeper, harder. But even as I climax alone, it's only hollow satisfaction. Is this normal? Should I talk more with him about monogamy?