"And no one puts new wine into old wineskin; if he does, the new wine will burst the skins and it will be spilled, and the skins will be destroyed. But new wine must be put into fresh wineskin. And no one after drinking old wine desires new; for he says, `The old is good'."(Luke 5:37-39)Renewal requires a full change; a complete turn around by burning bridges. The path to righteousness is really hard because we are enveloped with easier and physically satisfying alternatives. These hinder the total conversion that we desire. But Jesus clearly stated, when we need to reap the glories of a renewed life, we should leave behind our old ways, sins and earthly desires alike, and live a new life completely or else we will "burst."
The last line of the verses tells us that many failed by accepting the notion that old ways are better without trying the new. Jesus promised, and many attested that life in Him is rewarding if you just believe Him hook, line, and sinker.
If you doubt, don't doubt without bothering to try. There's no harm in trying anyway. Have a relationship with Jesus now.
"Everything is vague to a degree you do not realize till you have tried to make it precise." Bertrand Russell