Remember this is a really emotional time anyway. Your body is suddenly overflowing with hormones so every thought whatever it is will come out extreme.
Its normal to be terrified when you become pregnant. Both times I was actively ttc and both time when my test turned positive my first reaction was a wave of utter panic. Its a massive deal, however prepared you are, its not at all unusual to feel this way.
Then there is also the fact you are worried having another child might affect your other children, that again is completely normal. I felt really bad when I fell pregnant with my second baby, i kept having these guilty feelings, like somehow this baby will cause me to neglect my eldest child. It didnt, and you learn how to juggle, if anything it brings more joy to the family as your older children suddenly have a new buddy, but your original concern is normal.
Do your children get on with your current partner? Have they actively expressed the fact they would hate you to fall pregnant? Is your partner happy with the news?
Your life will obviously change a bit, but its only an adjustment.
Dont be too harsh on your self, you havent had a baby in 10 years, its all new really. But the good news is you are pregnant !