Am I falling alot on my snowboard because it's a new and shorter one than my old board?

I've been snowboarding alot the last two seasons and I was using a used board that was supposedly not fitted for me (past my nose). I just recently bought a brand new one, but this one is much shorter (up to my chin). When I tried it out a few days ago, I kept on falling and it felt really weird. The sales ociate told me that this new board is suppose to be the right size for me. Anyhow, I slammed really hard on my and now I'm in pain. Should I have gotten the same size board? Is it because this one is shorter and I just have to get use to it? Please help. Thanks!