*hugs* It can be very difficult to have someone address things that may be staring us in the face. But, believe it or not, the fact you are honest enough with yourself to say "Yes, you're right, it is a problem" is a GOOD sign. However, finding the motivation to DO it is the tough part. Because, if we were able to do that, we wouldn't BE in this situation. Right?
Are you seeing a therapist? Have you talked with someone about your causes of unhappiness, depression, etc? Have you talked with him about it? It can be very frustrating for a partner to watch their lover and friend devolve.
I often have this happen in relationships where I am single, vibrant, and secure and then I lose all that confidence and boldness in a relationship. Is it the relationship that caused you to lose that oomph? Or, is it something else?
Have you considered going to couples therapy? That may help too. Have you opened up with your partner about why you're unhappy? Have you come up with a game plan together?
And, the big question -- do you want this relationship to work? If so, why? And, if so, HOW? You can't will something to be -- you unfortunately have to put in an effort too. Which can be daunting and overwhelming and tough.
Ask yourself if you are there for your partner too. Is he the rock of the relationship? Do you do nice things for him? If nothing else, I find that even when I am feeling yucky and fat, if I do nice things for my partner (a compliment, buy him his favorite snacks, a kiss just because) it helps me feel a little better too.
Hang in there!! You know you've got support here.