“I prefer quantum physics, mechanical physics is too difficult”
Real astronomy and astrophysics are extremely computational with high level, hard core math, physics and CS all playing a role.
It’s not at all like what you see when celebrity scientists like Carl Sagan, Neil deGrasse Tyson or acclaimed actors with god like voices are telling dumbed-down dreamy quasi-religious string theory/multiverse timey whimey stories on PBS.
If you’re interested in astrophysics in terms of its popularization, you might be better off aiming towards science journalism, or museum educational careers.
If you want to do real astrophysics, you’ll have to get used to the idea of years and years of psets in hard core math and physics classes followed by even tougher psets, qual exams and a thesis in grad school.
A good into to the math of quantum text book by Peter Woit was published a couple of years ago. https://www.springer.com/us/book/9783319646107
That gives you a good idea of what his intro to quantum math course is like at Columbia University. A strong grounding in proof based calc, multi var, and intro - mid level college physics helps.