Happiness and sadness are intrinsic parts of life; both exist in the experience of life. The harder you try to be happy, upbeat, and positive, the more you will tend to feel sad, down, and negative. Don’t try to feel anything other than what you do and accept how you feel at all times.
During the times that you want to feel better, though, it’s helpful to take some time to reflect and be present. It’s so easy to get caught up in what’s not working, so reflect on what is working, what is good, and what you have that you couldn’t live without.