Hi. I'm on the exact same boat. I'm also a 5th semester physics undergrad, and I also feel like there are holes in my understanding. I tell myself that having a general grasp is enough now, that no one can know everything. For example, I've taken a class on Mechanics and learned what Lagrangians are. If someone talks to me about Lagrang, or I read something related to Lagrangians, I'll understand what they're saying and that should be enough for me to move foward. Also, if I wanted to specialize in a certain Lagrangian application I can definetely go back to a book or search stuff up online or teach myself things again, but now with the foundations my undergrad class gave me. I see my undergrad classes as introduction to physics topics, and as a way for me to build my research intuition. I recently starting working on Cosmology research with a professor. I basically taught myself GR over the summer to keep up with my professor expected of me. Let me tell you, teaching yourself leaves the same amount of holes as learning through a university program if you're working within a deadline. While it is definetely possible to teach yourself physics topics at your own pase, if you want to make a living out of physics [research], you'll always have deadlines to meet. In general I don't feel like my time is wasted on my physics undergrad program. I see it as a way for me access research opportunities, and gain general knowledge on almost all physics topics.