You really can't know for sure if you're clinically depressed without the professional opinion of someone trained in mental illness.
I sense that you feel some stigma or reistance to having that label? It's not a dirty word. You don't need to call it "the d word". It's no more shameful than any other disease. If you had an infection that didn't go away, you'd go right to a doctor and get antibiotics. So if you have a mental/chemical imbalance that isn't going away, why not get treatment, cognitive therapy, or medication? There is nothing wrong with getting help!
Recognizing you're depressed also doesn't mean you're permanently disabled. Depression can come because of grieving, extreme stress, or other serious problems in life. You list an possibly-unsatisfying long-term relationship, lack of close friends, and feelings of being isolated -- all environmental issues that CAN be changed. Maybe you have good reason to feel discouraged? A good therapist will help you recognize this and should teach you coping strategies, give advice on behavior modification, and assist you in becoming more self-aware of your deeper feelings.