Well... Could you elaborate ?
What do you think to yourself when lonely? Do you enjoy time on your own? Are you ok being different than him or struggle wanting to be same? What do you guys do before he leaves to be with his other partner? When he gets back? Is it a steady partner or a new one each time? Is it at night or in the day? A few hours or days? Stuff like that.
Perhaps articulating the details could help you see or narrow down the things to work on.