I can give you a perspective of being a PhD scholar. You will come a lot of confusions, self doubt, and all the peer pressure like whether i am worth it or not for a PhD. These things are common but remember if it was easy everyone will be a doctorate . About your subject specific doubt no one is there who knows all the things so don't focus on getting all the information rather narrow down your subject expertise and perform well in that and with time you will learn all the relevant things related your subject, and remember one thing its better to be master of one rather than knowledge of all master of none . Hope this helps you a bit.

Очень важно, что бы вам нравилось заниматься научной деятельностью. Ваша цель должна быть не получение научной степени, а занятие наукой. Продвигаясь в этом направлении вы будете формировать багаж знаний, который в конечном счете приведет вас к желаемой цели.