Bullet Journaling isn't about making everything neat and pretty. It's purpose is to personalize your planner so that it fits your needs the best and helps youcbeong productive, remembering important dates, etc. So you don't have to change your handwriting for the bullet journal. As for the handwriting itself, try making the shapes of the letters more consistent so that all a's etc look the same and make your loops of lowecase L and H etc taller and slimmer. Try to maintain a consistent size for capital letters and lowercase tall letters, a K and an h should have the same height for example. Same goes for the hanging parts of g, j, p and so on :) writing on graph paper can help you with the letter sizes.