Guilt and other mommy feelings

This parenting thing is HARD. Not just a little bit of a challenge but a cram for an entire semester and then still feel like you are winging it kind of thing... really freaking hard.
We have been shut into our house for days- weeks actually- because of winter onslaught and I am tired of all day arguments with 3 year olds and non-napping infants. It doesn't matter that I know better than to argue with someone without a fully formed frontal lobe, the kid really knows how to push my buttons. And he throws one heck of a tantrum.... so do I for that matter (I guess he is paying attention to at least one thing I say/do). Discipline has started to feel like a chore for me and does not seem to be working at all. I know I am not alone in this and truly I understand that the age has everything to do with it. One of these days my sweet faced boy will actually be "sweet" again but for now I just go day by day. Also he is completely jealous of his mommy hogging and perpetually enchanting baby sister! It is impossible to even out the attention since I nurse her 4 times during the day and she is in that clingy separation anxiety stage. So how do I make him feel loved and supported when half the day I am attached to his nemesis and the other half of the day he is in time-out for smashing her face into the floor or yelling at me??
Did I mention this is impossibly hard?! I am the primary disciplinarian because I only work part-time but at the same time, I feel like it is up to a mommy to be the ever present rock of support for a kid- the one who kisses the ouchies and tucks them into bed- acting a mom is an oxymoron for sure.
We have a deal that he gets 3 songs when he gets tucked in bed but in return he has to whisper and be quiet to not wake up baby sister (see earlier post)- entirely a work in progress. Tonight he ran yelling onto his room, slammed open the door and jumped into bed with a war cry. In return I told him (gulp- i knew the firestorm that would ensue) he was going to bed without a song because he was not quiet, that I loved him and would see him in the morning. Enter legendary tantrum, complete with screaming at the top of his tiny lungs, an awakened screaming baby to match and another headache for mom and dad. I retrieved the baby and my husband went to explain to him why it went down like it did. Hard enough, but then the guilt sets in. I am nearly in tears myself because I am again with the other baby while he cries himself to sleep in the other room BUT to go sing him his song would certainly be counterproductive.
Anyway, I just wanted to lament the feeling of a loss of tonight's happy little bedtime. The only consolation is that the under-developed, tantrum-prone neuroanatomy of a three year old will probably not remember me forever as the viscious mommy who ditched him in his time of need for his crying baby sister... but I will remember :(