You may have a few vices you would like to be rid of, but it is important that you don’t try to break all your bad habits at once. You’re not superhuman and trying to kick a smoking habit, give up chocolate and stop biting your nails all at the same time is probably going to be hard to achieve. Instead, prioritise and pick the habit you most urgently need to change first. Also, try to set a definite goal to work towards rather than a vague “I’m going to stop being so unhealthy”. Identify what exactly it is you need to do (or stop doing) and make that your focus.
Find a solid reason for breaking your habit that really means something to you — such as improving your health, appearance or relationship — and keep your goal in sight. Reminding yourself regularly of the bigger picture and the reasons why you are doing this will help you to carry on and stay strong in moments of weakness.