Who knows? Sometimes these things are mental and sometimes there's an underlying but hard to find physiological cause, such as blood sugar imbalances or hormonal imbalances, including thyroid problems. Whatever it is, nobody can diagnose you over the internet, although many will try.
It sounds like anxiety, depression and maybe bipolar depression, but I'm not a doctor. I do have anxiety and depression and I totally get how you feel. It *****, doesn't it? :/
I know how you feel, i feel this way quite a lot. You just have to know that these thoughts will end, what i usually do when i feel this way is i would think about what i have instead of what i don't have, and i would always think about the people that do care about me. I would remember how beautiful the world actually is. And all the stuff you can do in it.
I think you are going through depression's early stage, along with proper treatment you can come out of it. Never loose hope!
Symptoms of depression may include the following according to the National Institute of Mental Health, :
Difficulty concentrating, remembering details, and making decisions
Fatigue and decreased energy
Feelings of guilt, worthlessness, and/or helplessness
Feelings of hopelessness and/or pessimism
Insomnia, early-morning wakefulness, or excessive sleeping
Loss of interest in activities or hobbies once pleasurable, including sex
Overeating or appetite loss
Persistent aches or pains, headaches, cramps, or digestive problems that do not ease even with treatment
Persistent sad, anxious, or "empty" feelings
Thoughts of suicide, suicide attempts.