The transition from undergraduate to Engineering is Not easy at all. In the past 15 years, we have seen brilliant students struggling with core engineering subjects. These are Statics, Dynamics, Mechanics of materials, Statistics and Probability, Thermodynamics, Fluid mechanics, Structural Analysis etc.
All these subjects require a very sound understanding of Physics and Calculus. Physics is the mother of all sciences, and mathematics is their language. At the engineering level, calculus is the core of all mathematics.
If the foundation in Physics and calculus is not good, then one can not learn the above subjects. That is the reason why many students struggle with these core engineering subjects. So if one wants to improve these subjects, then start with Physics and Calculus first. After you learn these two, the others will be a lot smoother.
Physics and Calculus form the base, but the core Mechanical Engineering subjects start with Engineering Statics. It is also called Mechanics-1. Then comes Engineering DYNAMICS or Mechanics-2. Then comes Mechanics of Materials (MOM in short!) or the Mechanics of Solids. Some people also call it Strength of Materials or SOM. These three subjects are the basis of other advanced engineering subjects.
Once proficient with the above three, one can start learning real-world engineering application subjects like Design and Manufacturing. Statistics and Probability is unrelated to all these subjects, but everyone needs to study them. It has such vast applications that even non Engineering streams like Business and Finance study it. There are many other subjects like Thermodynamics, Fluid mechanics, Structural Analysis.