How can i just focus on the present?

I mean i think im crazy as hell or something, because i'll feel great and positive then after awhile say i get bored not doing anything sittin down or watever, i start to think about stupid things of the past.. things that i regret and things that dont even matter anymore.. and i dwell on this for no reason.. say im just sittin watching TV or something i'll start to get myself depressed thinkin of things i regret and things i could have done better in the past.... how can i just like block these thoughts out and focus on the good things in life?? like the future and the present