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
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:
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 :)