"What did you do if you knew you wanted to model something, but didn't know how to make it or just make it look good?"
Like we, devs, do with everything else related to our games... model another thing to replace it or model something similar and give it another context (considering that you just can't do it).
We almost never can fully do the game we first imagined, we have to sacrifice stuff and change stuff to fit what we best imagined into reality. Triple A masterpieces from the past and today also "suffered" and took advantage from this, taking advantages from limitation.
"Was learning how to 3D model things yourself hard/worth it?"
Worth it I think it always is... I like to think that knowledge is never enough, it raises the set of skills you have.
Like knowing a foreign language, even if it is a language that is really not popular, knowing it will always be one of your unique skills (considerating how hard it is to learn those stuff that takes years of practice).