Here are the suggested answers for the blanks:

1. **A. should**
2. **C. harder**
3. **D. lots of**
4. **C. weak**
5. **B. some**

So the completed text would read:

"My name is Libby and I'm 12 years old. I never manage to go to bed early during the week. I know I (1) **should** use my phone or tablet late at night, but I always do. I find it (2) **harder** to fall asleep, but I don't know why. I often miss breakfast, so I'm hungry by the morning break. In my school, you don't need to go far for unhealthy food. There is a vending machine with (3) **lots of** sugary snacks and fizzy drinks like Coke or soda. My mum says I should eat some fruit or yogurt for a snack, but I keep forgetting to bring it to school with me. I also feel (4) **weak** and have no energy during the day, which is really annoying. I really need to get fit and healthy for the school bicycle race. What do you think I should do to prepare for the race? Please give me (5) **some** advice on how to become a better me."