1) Stop invalidating how you feel. Just because you feel like other people have it harder then you doesn't mean that you're not allowed to feel sad or down. Personally I find one of the most demoralizing things is feeling like your emotions are invalid, and feeling guilty for feeling bad only makes things worse. It may not seem obvious but everyone has their freak outs and times where things are rough and that is ok.
2) It may sound weird but try exploring how the emotions feel. Do they feel different in different situations? Is it more pronounced in certain situations? Does the feeling have triggers or feel connected to any aspect of your life? I find simply thinking about how it feels can help me determine the source.
3) Find things to look forward to. It doesn't have to be big but it's important to plan things to look forward to, it helps a lot with getting through the rough times. Even something as simple as a nice dinner every once and a while, going out with friends, or even a cold beer after a days work.
4) Seriously think about what makes you happy and what you want to do. Don't let other people define happiness or success for you because ultimately they are not the ones who have to live your life.
Hopefully that helped. Feel free to send me an email (in my profile) if you want, I'll try and help out if I can.