Through a lot of hard work. Have you considered cognitive thereapy? (CBT)
Are you seeing anyone? Therapist/psychologist/counsellor?
Conquering paralyzing negative thinking is not as simple as just saying 'okay I don't want to do this anymore' and *poof* it's gone. Since a lot of it tends to build up over time - decades, for some people - it takes the same kind of diligent effort to face it and put it behind us, or at least manage it in a way where it's not so counterproductive in our daily lives.
Another thing that's cheesy and effective: daily affirmations.