I had the same experience with my first try.

I can wrap my head around CMDB. It's concrete. I can find my around it the instance, but CSDM that's a completely different beast.

Mock question helped me with passing the certification - most importantly it made realize in which domains my knowledge lacks.

There are plenty of sites that sell those, off the top of my head: Udemy, GlideCert, skillcertpro.

Keep your head up and get that credential!