I felt the same as you OP: as though I didn't have any reserves of maternal affection stored up to give, and that really concerned me.
When I had my own kids I would always ask myself: "What would a good, loving mother do/say in this situation?" and do that. Always take a moment to think before you react. I know that's the question you're asking here - what does a good mother do? - and you don't have that blueprint, so try to put yourself in the child's position and think about the most helpful reaction you could get from a caring adult.
I also reminded myself that it takes the same length of time to respond with patience to a child as it does to snap or shout at them. The outcome - both short-and long-term - is much better if you're kind.
Patience. Lots of it. I had the word written on my hand in biro during the toddler years as a constant reminder. They're small, and they will try your patience, but not to wind you up deliberately. It's because they're learning, and they need help - lots of it - and love, and care.
And apologies. If I ever had a day when I'd snap from tiredness and become unreasonable, or impatient, I'd go and apologise afterwards, and we'd talk it out.
Parenting books have helpful guidelines. 'How to Talk so Kids Will Listen and Listen so Kids Will Talk' gave me good ideas on how to be more patient and understanding. Better books may have been published since mine were little: loads of good suggestions in earlier comments.
It is HARD when they're little, and shrill, and sticky, and don't sleep, etc, but it is so worth it in the long run. For what it's worth, mine are now teens, and are genuinely lovely to live with, every single day.
I felt exactly the same as you when mine were 6 and 3 too. It's not too late. The fact that you want to be the best you can for them means everything. X