A lot of what you said rang true for me. It does sound like you are depressed, but perhaps you are able to work things out and learn to improve the way you feel. Never feeling content is something I have, and also, like you, it is in relationships as well...I think I dont want that person, then when she is gone I am a mess. That lack of contentment can lead to some daft decisions for sure.
Acheiving a feeling of contentment is probably something most people would like to do yet havnt done, but when it gets to the stage when your crying, lying in bed and feeling worthless, it is time to try to start getting yourself better.
You probably feel like you are making it up, making excusses for yourself or are just complaining too much....but the way you are feeling is unpleasent, and so you are right to try to look for help and consider that you may be depressed. You might not be due for a severe episode anytime soon, but it is right that you acknowledge that you are feeling this way, and I think you are sensible to want to change this situation, so that it wont get worse. The mistake I made is feeling just like you, but I kept ongoing, telling myself I was beeing soft, and over the space of a few years I managed to get very ill.
Take care,
Steve