I think it's good that you're working through the sadness and pain you're feeling.
In general, I find good music and walking to be the main things that help me. I have used affirmations by Louise Hay for many years. She believes that positive affirmations can set your mood for the day and change how you feel. I will PM you a link.
Since you want to be in a more positive mood for the job interview, maybe you could try writing/journaling. Put together your thoughts about the mood you want to show at your job interview and see if it helps.