I think you need to try and let go of the associations personal writing has with childhood and approach it from a more adult perspective - you're not obliged to write in it everyday or address it to anyone. A date and a few lines can be all it takes to make the practice feel useful. At first it might be better to adopt a more formal approach, like using it to keep track of certain symptoms/moods. If you progress to more in depth writing but feel comfortable with it therapeutically, why stop? Some good suggestions from above posters, good luck.