I would recommend that first of all, you define 'happy', because it means many different things to different people. Here's an exercise I'd use to find out what to do in your own music
Find examples of the mood you're trying to set. Write down as much as you can about the tempo, the instrumentation, possibly the scales and types of chords used(from sheet music or something notating it) and find patterns
write down the attributes of your own music. What attributes does it share with the mood you're trying to set, and where does it differ?
Next, rewrite sections of your melodies to include the attributes missing from the original. You might consider adding one attribute at a time and seeing how the effect changes, or using melodic 'sketches' to try out all the different permutations.
From here, if you've included all the attributes and your music still doesn't have what you're looking for, begin the process again, this time looking for more attributes which your piece is missing, fresh with the knowledge of what you've already considered :)
and above all, write fearlessly ;)
I use the word attributes specifically because you're not trying to copy the music, but a particular element of the music. Focusing on the attributes means you can apply it to your own style without just ripping the original off.