I think "grandpa" works better because it sounds more personal.
Can I switch out "experience from different jobs" with "experience working", and should it be "making a choice" or "making my choice"?
I actually think the better, succinct way is:
"my grandpa advised me to
get some experience
from different jobs before making a choice."
And you don't have to be too picky with the word choice. I think "a choice" is perfectly fine.
"I would like to major in Communications at CU to gain more knowledge
about a field I wish to continue working
in "
Can I write CU, or should I use the full name "University of Colorado at Boulder"?
You should always assume that the reader doesn't know the abbreviations. So it is better to write down the whole name.
This final sentence, as you know yourself :), is awkward. Let me try to fix it:
"Even
if it has takenthough it took me four years to get here,
thanks to my grandpa, I finally know what I want to study
, and I have my grandpa to thank for that ."
From there, maybe you can improve your conclusion even more :)