Don't give up on your GCSE now! If you're annoyed that you didn't take school seriously it won't help you to fail to take this seriously either. Keep doing the work, once good grades start coming in you'll feel far more motivated to continue and will, quite rightly, feel really proud once you've achieved the GCSE, that in turn will help build up your confidence, which will in turn make you realise the real options that are open to you and give you the courage you need to find work that you are happier in.
I agree with Pa__ul that it's anormal thing to feel around our age. I left uni 3 years ago and have only just pulled myself together and realised that only I can create oppertunities for myself and only I was holding myself back from getting out of a job I didn't like. Now I have plans, and while it'll take 2 years for me to becaome qualified, I'm willing to wait those years out knowing it will mean me being what I want to be. You can do that too, there's loads of oppertunities out there, it may mean working extra hard, or being patient, but in the end it's worth the patience and the work.