I previously had a follower mod on the original Skyrim, but when the special edition came out I couldn't get a new follower like Lydia. She kept saying you already have a follower. Do I have to start a new save file?
It sounds like you tried to port a modded original Skyrim file into Skyrim SE, which has been warned against. See this article which is aptly titled "How Skyrim Special Edition works (and doesn't) with your existing saved games and mods".
From the article, an email directly from Bethesda says the following:
"Only use original saves that have never been used with mods. Do not use your original saved game if this error appears when you load it in Skyrim Special Edition: This save relies on content that is no longer present."
Additionally, the owner of Nexus Mods provided the following quote:
"Unfortunately, we haven't had enough internal testing to give you a definitive answer on modded save file compatibility. According to our community of mod authors however, the general consensus regarding compatibility of modded saves is a resounding 'no.' Though there is a remote chance that very particular save files which only depend on mods which have a SSE equivalent installed may work, this will not be relevant to the vast majority of mod users because their modded files will have one or more dependencies with a currently incompatible mod."
"Our advice for mod users is to expect to start fresh when it comes to playing SSE."
There really is no solution to an issue caused by porting old modded files into Skyrim SE other than to start a new file without mods.