My obsession with weight has officially created a rift between me and my girlfriend

My boyfriend tells me he loves me just the way I am, but honestly, I can’t seem to stop comparing myself to thinner girls. When we go out and I see someone fitter than me, it totally brings down my mood, and I end up picking fights — it’s unfair to him. I just can’t help it. I really don’t know how to overcome this lack of confidence; I’ve reached the point where I hate myself.