Just do it, it's a google search away from you.
'Over thinking' and 'self destruction' sound like symptoms of depression/anxiety. If that is the case, you will eventually have to seek out a professional for diagnosis and treatment. Importantly, you can't rule out mental health issues without consulting a professional, especially when these problematic patterns of behaviour are present in so many areas of your life. If you do have a mental health issue then it's likely that you won't be able to fix the issues you're experiencing without first treating it, which is why consulting a professional is such an important step.
Independently of the existence of possible mental health issues, a psychologist will be able to equip you with tools (cbt etc.) to facilitate the changes that you want to make, and support you through them.
As a mid 20's white-male who has experienced a somewhat similar situation, I wish that I had sought out professional help sooner. If I had, repairing the damage caused by un-diagnosed major depressive disorder and social anxiety would have been a much easier task.
If I were the OP, I would tackle the problem by scheduling more time every week being active, and being social. Fortunately, it sounds like he has plenty of opportunities to do so, so it's just a matter of consciously spending less time alone.
It's difficult to have a “dark night of the soul” while you're chatting with somebody in a steakhouse, or tossing a frisbee around.
Current NHS figures suggest about 60% success rate, and that's on a bare-knuckle minimum provision model.