As someone who has dealt with / is dealing with PTSD and depression.
1) Seek medical advice. Medication can help.
2) Tell people that care about you how you feel. It is difficult but helps them understand your mood or demeanor.
3) Focus on people that you care about. Look at ways of making THEM happy. Happiness can be infectious.
4) Get a dog.  ( Research the breed so it suits your personality and lifestyle ). Dogs are emotional beings but also live a simple life. It takes very little effort to make them happy and they can be immensely loyal.
5) Exercise and get plenty of sunlight.