Hi there,
Sounds like you have depression and maybe anxiety. They are two separate things, both of which I have had on and off for years. They are like that, they can go away and come back over and over. You don't even need to know specifically *why* you are feeling that way. It just happens sometimes. I also went through several years in my younger days where, as you described, I would literally feel empty inside. I wasn't happy, I wasn't depressed. I was working yet had no goals, and couldn't understand how to set and achieve any. I didn't even know what I wanted. And it bothered me constantly to feel empty. I'd rather feel sad than empty, because at least the sadness was identifiable. The emptiness was just a gray void that made no sense. My grandpa died in his sleep of a heart attack, and I used to lay in bed at night wishing for the same thing, that I would just go to sleep and not wake up, even though I was way too young at the time to have a heart attack.
I too have been in a relationship with someone who was bipolar, so I can imagine what you must have gone through. I lived with my college boyfriend for 3 years, he was bipolar, and at times very unreasonable and uncontrollable. He went through paranoid periods where he refused to take his meds for it. When he was happy, life was wonderful, but when he was upset or mad, life was hell. Eventually I ended the relationship and moved on to something else. I once had a shrink explain to me that it is very difficult for a person with bipolar disorder to calm down and be reasonable, due to the chemistry in their brain and how it works incorrectly. Where a typical person who is not bipolar might take a few hours or a day to get over an arguement or mishap, a person who is bipolar will take days or even a week or more before their brain chemistry balances out again. Makes sense.
Many people go though periods where they feel life is overwhelming and they don't know how to achieve what they want, and that they are all alone and nobody loves them. This is very common. At least you can have some consolation in knowing how much others feel like this and don't say so. Sleeping alot and not eating much (or too much) are depression symptoms. Feeling panic and/or anxiety is also fairly common and it isn't unusual to have depression and anxiety at the same time. One can cause the other and many people experience this.
What you can do about it is truly up to you. Some people find it helpful to find a therapist and start taking meds for it. There are many of them available. Personally I have never liked any of the results I got from antidepressants, and prefer to take a more holistic approach to my problems, such as making myself try to live as normally as possible; getting a normal amount of sleep, eating good things when I should, taking vitamins, and exercising when I can make myself do it. I know from personal experience that exercise is a very powerful tool against depression. While still in college, I was once living in the very northwestern corner of California next to the ocean. It gets very foggy and rainy there and there isn't alot of sunshine. Hence I got horribly depressed very quickly, and stayed that way for months. One day my roommates got me to go to the weight room with them to work out. I worked out longer than everyone else, and felt awesome for 3 days straight afterward. As long as I made myself exercise, I didn't get depressed then.
This is not to say you shouldn't seek the advice of a doctor or therapist for your problem. Just because it doesn't work for someone like me doesn't mean it won't work for someone else such as yourself. Everyone is different. Eventually you will start to learn over time what works for you and what doesn't. Some people just see a therapist for advice and help without getting prescription meds.
But the best advice I can share that you can do right now that won't hurt at all, is to get up and make yourself do something. The only piece of good advice the worst doctor I ever had gave me was, "Go outside and go for a walk if you feel depressed. Even if it's just to the end of the block and back." I took his advice and started making myself walk a little farther each day unitl eventually my daily one-block walk turned into a daily 3-mile run.
Be easy on yourself. It is hard to get things done when you're depressed. Even if you just make yourself get out of bed and function, go to work or school or whatever you do daily, tell yourself that you accomplished something good. When you are depressed even baby steps can be hard. I like to write my accomplishments down even if they are small, such as "Today I washed the dishes". There is a saying I like to live by to help me not feel like a failure at everything I do, "Let whatever you do today be enough".
I hope this helps a bit. At least know that you are definitely not alone, that many people feel like you do, and there are ways of dealing with it, provided you can see them through the darkness. Best of luck and health to you.