I feel sad all the time. i don't remember the last time i felt good about anything or happy. is this some sort of mental condition?

Could well be: First ruling out medical conditions such as low thyroid, cardiac conditions, anemia, etc is wise by consulting with your physician. If not medical, consulting with a psychiatrist or clinical psychologist for possible depression woudl be wise for someone with such complaints. Good news is that it is likely very treatable with psychotherapy and/or medication -both are often more effective in combo.