>>542104532
first things first: if you can't do a long combo, do a short combo. it's fine.
second: long combos aren't one million hit long thing that you have to memorize all of, it's a bunch of bite sized combos strung together. once you've messed around and learned what strings into what, you'll become able to intuit things more and more easily.
for now, just do simple stuff:
light>medium>heavy>down+heavy>jump>light>medium>heavy>special move
once you're comfortable doing that, find a way to end it with a super and you'll start doing actual damage.
finally, don't worry about the combo trial combos. the longer ones are meant to be gameplay challenges, not necessarily "stuff that is optimal/important to be able to do." just take things one step at a time and have fun learning, because there is a lot to learn and you can always improve something
you can do it! i believe in you!