I'm usually content with my life but lately things aren't going my way. Everyone says I just need a girl to make me happy but it doesn't make sense to me, especially when I'm not happy how can I make her happy.
Your question has it the other way round man. You do need to find a girlfriend to be happy. Unless you're naturally born a selfish person, then you can live happily ever after with just youself, best pals and your possessions. Back to what I was saying, in most cases, a gf's presence would be what makes the dude happy on the inside of which would overflow on the ouside. This motivation then gives him the strength to keep his girl happy by doing favours for and being sweet to her volitionally.
Both people need to be happy in order to truly appreciate each other and the relationship they're in. If you're unhappy then you place the burden of for your own happiness on to someone else and that's not fair because it means you are expecting them to make you happy all of the time and take no responsibility for your own happiness.
It becomes tiring having to deal with a partner that places too much burden on to you and then get angry and blames you for their inability to take charge of their own happiness.