Basically what this means,

For whosoever will save his life shall lose it; but whosoever shall lose his life for my sake and the gospel's, the same shall save it. For what shall it profit a man, if he shall gain the whole world, and lose his own soul?

You cant gain happiness by possessions or money or sex. Every celebrity has proven this. Without God we have nothing.
When we get to caught up in the things of this world, and dont have God, life is meaningless. We lose sight of what is important, and we are no longer truly happy.
When we say to God, i give my life to you, your in control, everything i have youve given to me anyways, if you take it away its fine. We are "Losing our life" for Gods sake, and in return God blesses us, giving us a great life. Losing your life, just so your can find it.
And the guestion about Hell contradicting itself, its not a contradiction.
When God comes back, which is hopefully soon, hes gonna judge the earth. The people that beleive in him will be raptured (Taken up to heaven) Everyone else will stay on the earth and it will be chaos. But God will give everyone a second chance. So when he destroys the earth 5 years later, he'll take back to heaven the ones that believe in him, and the ones who dont, will go to hell.
He'll then create a new heaven, a new earth, and a new hell known as The River of Fire, that will be even worst.
You cant just take peices of the bible, you have to read the whole thing.