If there is something that you used to regularly do because it put a smile on your face, feel good and proud of yourself. But these days you're not sure if you've still got it in you to do whatever it is - try thinking and feeling these things to get you doing your thing(s) again - that make you feel happy.
Firstly, you might have been told when you were in your younger years that you weren't clever or good enough (your: looks or soul) to do it / them. Write down everything you've done well and achieved - read them out to yourself while you look at yourself in the mirror, and say "I proved you wrong by achieving what I wanted to achieve and I was good at it - so screw you, you over negative horrible person".
Then do the thing that you know in your heart of hearts that you were born to do. You'll feel endorphins (feel good hormones) rush through your body / system when you let yourself go - get really into it.
E.G I love music - dancing and DJing are things that I do that I'm good at. They make me feel :very happy and proud of myself. When I make myself stop thinking about anything other than the music - I really feel & hear the beats to dance and DJ well to. When I make myself be in the present moment, I feel very: proud of myself and happy because feel good hormones Endorphins rush through my system and I feel so good I often cry happy tears of joy.
If you let yourself do whatever you want to do (so long as it's legal), when you need to lift your mood and you need to feel happy like you did in your younger years. Make yourself write down the things you want to do, then be in the present moment - put small life details out of your mind e.g. what food needs buying etc - do your thing(s) that makes you love being alive - really let yourself enjoy this feeling. Tick off the things you wrote on your 'to do to be happy list'. Smile when you do this and let yourself feel proud of yourself - GO YOU!!!