You kind of answered the first question already-because it’s easier for you to write from sadness.
I don’t think you should be very worried. Sadness is a great writing tool, actually. I think more passionate, real writing comes out of it than from happiness.
As far as tips go…maybe try to write at least one poem or piece when you’re happy a week. Just one piece a week, so you don’t get overwhelmed but you’re still branching out of your comfort zone!