God keeps giving you these divine pushes, and all you have to do is go along. Through the sacrament of penance, through your reconciliation to God, you receive sanctifying grace. It means death. Mortal sins are deadly sins because they kill off this supernatural life, this sanctifying grace. Venial sins don't destroy supernatural life, and they don't even lessen it.
Mortal sins destroy it outright.
The trouble with venial sins is that they weaken us, making us more vulnerable to mortal sins. When you lose supernatural life, there's nothing you can do on your own to regain it. You're reduced to the merely natural life again, and no natural act can merit a supernatural reward. You can merit a supernatural reward only by being made able to act above your nature, which you can do only if you have help. To regain supernatural life, you have to receive actual graces from God.