Can i suggest you have a look at what you are or not eating. There is research indicating not enough B group vitamins, sluggish bowel, and dehydration can certainly contribute to a change in mood. For some, me included, there are certainly some foods and drinks that can quickly change a mood for the downer. Make a diary of foods and drinks you consume, activities you do and people you hang out with and see if there are patterns. Mind that chemicals in foods can be misleading and hidden and so it is an area to explore with a trained dietitian/nutritionist who can put you on the right track.
Exercise is important. Between the fresh air, blood moving with more oxygen to the brain and happy hormones such as oxytosin floating through your body we tend to develop a different perspective of our situation. Exercising with a friend or buddy is even better as we bond and start to feel better just having a happier person around us.
These are just a few tips and if you need more specific guidence be sure to contact a professionally trained councelor or more.
To your health