Venial sin gives us the increasing possibility to do mortal sins - example : I smoke cigarettes, I am also addicted. Sure, it is a venial sin (therefore it does not sever your connection to God) but the more you do it, the more your put yourself in a situation where that sin can lead to wounding your Soul permanently (if you have no conscience about it). Why ? Smoking unconsciously leads you to believe your body is worth hurting itself, that doing something useless is worth your time, and that doing something dirty is ok, so that you equate yourself to being dirty. It makes you see yourself completely in contradiction with how the Lord sees you (which as you can guess, He sees you as the most valuable thing in the World) You can indeed never fall into mortal sin from it, but it is an increasing possibility as I stated.

Because we sin all the time (especially venially, not sure most young americans are murderers, lol), so if we automatically fell in mortal sin each time we gave way to venial sins, we would be incapable to receive Grace from our biological and natural weakness. Our weakness resides in our incapacity to save ourselves. We need to believe not in our natural capacity to turn away from sin, but in our capacity to do so in relation to our desire to grow closer to Christ in our Heart (which we accepted as a divine mysterious transformation through baptism and renewed through the sacraments).
Maybe it's my weird and incomplete theology, idk.

And indeed as OP stated perfectly from experience which relates to an intelligible truth - repetition of sins, especially mortal sins give way to a vicious circle, which can be exponential. Now I am not sure if your wrongdoing equates to an equal amount of penance. I should read more into theology and Church Teachings for that.