What's the hardest part about being a parent?

The hardest part about being a parent is baby doesn't come with a manual and we need to learn along the way through numerous trials and errors. E.g. kid's sleeping, food, education and etc. Also, I find it hard to divide my attention among my 2 kids and soon will be 3 kids. I always have guilt in me for not spending enough time with them despite I am a stay at home mother.