I think you're putting too much pressure on yourself, I'm sure you're a great nurse! You want to do well and that's important.
It's really not possible to know it all in the first year or even few years, after 10 years I still recognise my limitations, try to develop my skills and ask for help when I need it.
Likewise you don't need to know your specialty right away, there will be plenty of opportunities to change again in future if you want.
The experience will come, if you have support from your colleagues you will eventually become more confident with high acuity patients.