PNT is the most difficult theorem I encountered thus far

$$\sum_{d/n}\frac{\mu(d)}{d} = 1 -\sum \frac{1}{p_i} + \sum \frac{1}{p_i p_j} - \sum \frac{1}{p_i p_j p_k} + \cdots$$
Apostol, Chapter 2: Arithmetical Functions

Although I am doing M381 and M373 this year, my real goal for this year is ( read: was ) self-studying Apostol's Analytic Number Theory. I came to realize that a goal like that must be wrong if I want to go anywhere. It may look like a SMART goal but it is not. ( SMART goals are S(pecific), M(easurable), A(chievable), R(ealistic) and T(rackable). ) Since self-study projects are not finished with an exam, it is hard to measure if the goal has been achieved, the goal is not specific enough. After about 1/4th of the study year I changed my goal to: "understanding the proof of the prime number theorem (PNT)". Although the main purpose of Apostol's textbook is just that. I am now free of the strict planning I imposed on myself, i.e. March: do AANT Chapters 2 and 3, etc. I can focus entirely on understanding the proof no matter where the knowledge comes from. Once I understand the theorem I should have a new outlook on the Number Theory field and decide where to go from there.

I chose Apostol's book because it is used in OU Courses M823 and M829, of which I sincerely hope are still around when I am eligible to do them. I thought it was more or less the Analytical Number Theory Bible, well it is not, it seems. The classic work in Analytical Number Theory is the following book by Iwaniec and Kowalski.