If you feel down and need to brighten yourself up, remember that it is your mind that affects your mood. Thinking negatively, dwelling on bad memories or unpleasant events, worrying about the future and so on and so on, is going to make you feel bad about yourself.
Try a simple exercise if your 'glass is half empty' at the moment.
1. Realise what you are doing and acknowledge it.
2. Praise yourself for identifying that you are thinking negatively
3. Get out of that bad thought, now, just leave it behind.
4. Replace it with a thought about something you have achieved or something good you have done, however big or small.
5. Amplify this thought and give it time to reflect on what you did, the work that went into it, how kind or thoughtful you are to others etc.
Do this as often as you can, whenever you think negatively about yourself, your life, your day. Hold it for as long as you can.
Aim to replace your negative thinking with positive thoughts, and above all - to value yourself. Your mind can make you happy - its the most powerful tool you have.
I won't say good luck - because luck has nothing to do with it. You do.