This is not what the Catholic Church teaches. Venial sins do not send people to hell and do not require confession for forgiveness, although we believe it beneficial.
Mortal sins are an entirely different kettle of fish. Catholics believe they are the "sins unto death" mentioned in the bible. When you knowingly, willingly, and with full consent commit a mortal sin, you are basically pulling a "Lucifer" on God and will go directly to hell if you die, do not pass go, do not collect $200.
You have two ways out of this pickle. The first, and usual way, is to repent and go to confession. Et voila, no more Hellfire (although a chance of purgatory if you weren't repentant enough). It's quick, sure, and easy.
Option 2, not so simple. You must have Perfect Contrition, and the intention to confess the proper way first chance you get. Perfect Contrition means you repent of your sin out of love for God and sorrow at the horrible offense you have made him. This kind of contrition does not include repenting out of fear of Hell or because you've realized how awful the sin itself was (either of which is sufficient for confession). Achieving Perfect Contrition is Not Easy, especially if you've just made yourself dead to the grace of God by comitting a mortal sin. Why take the chance? That's why God gave us confession in the first place, so we wouldn't have to go around wondering all the time.
For supporting bible passages and quotes from the Church Fathers, check out:
and
Ed:
Eepsy has it right. This is the Church's real teaching.
Excellent points. Thanks for the links too!