Probably stems from not knowing if the next day could be your last. There are many people who get this feeling. I think that when you see something new, it reminds you of how long you've been alive. But then again, It could be something much deeper. I wouldn't know. I'm not a therapist.

Its just after I watch something new, the past felt like dream or memory and disappeared without a trace.

And whenever I imagine see the new stuff and the old stuff together, I feel like the modern matrix-like commercialism in today's media will overthrow the golden days of the past.

yet I keep telling myself the classics of the 20th century will always be better than modernism of the 21st century

Trust me. Our generation will always end up valuing the past.