Is it true that those who commit suicide go to hell?

This is addressed to Catholics I suppose... To me, there is something very wrong with saying that those who have gone through very difficult times and have not gotten the help they needed go to hell even after all that they have endured, while all of us fortunate enough to be "happy" go to Heaven for even more "happiness". Certainly every human being suffers moments of sadness in their lifetime, but for those who suffer unending amounts of it, how is it fair that they go to hell for trying to escape? I ask this because my uncle was in the Navy and wanted out and was refused, so he parked his car in the garage and died of carbon monoxide, as far as I know he was a great person, just troubled.