Depression or Grief? Please I need to help my best friend?

I adopted Cody at the age of 2. His behavior was shy and very timid. After a week Cody slowly seemed to be adjusting to our family well. He stayed very close to my daughter and son becoming very protective as well. My son soon moved out as well as my daughter. When my daughter was in school Cody would anxiously wait for the bus “somehow he knew the exact time of it’s arrival. He would begin the happy ritual of going outside to greet her walking up the drive. Soon she moved out this is when his behavior became saddening to me. He would not socialize with me and stayed in my daughters room just laying in her bed only to briefly come out for a quick bathroom break then back to her bed and lay. This I found heart breaking so I pleaded with my children to visit even briefly for Cody’s well-being. He is seemingly sad all the time and when he goes outside he seems to constantly look for my daughter and son. I am told that when I leave Cody howls and cries the entire time. To the point my ex-husband when staying one night called concerned to ask what was wrong with him. I cancelled my date and rushed home . Even now Cody and I are always together. He is aggressive and territorial so as responsible owners we limit his interaction with other people and animals. Beagles are not aggressive but Cody is! Cody is a mix breed of beagle and dachshund maybe this is his aggression, I do not know. How can his behavior of wanting to be alone in the next room all the time be explained ? How can I help him be happy again? Cody is also afraid of everything noises;people;etc. but when on a lead or in my home is very aggressive. I have seizures and it seems when a seizure occurs Cody will be under my feet literally. and upon having a seizure he guards me letting no one near me. Cody ,also has,seizures after a incident on a traditional collar and lead so he wears a harness now; I dislike a traditional collar around a dogs neck!! All I want is for Cody to be HAPPY again. Please help me?