Ideas/ opinions?

So my hair is an absolute disaster. It's puffy and frizzy and it never curls right. I've tried the curly girl method but it does nothing to help. I've used hair masks and conditioner all my life. My hair was down to my ass and I cut it to my shoulders but that only made it worse (and I regret losing my long beautiful hair)

So I want to get a haircut and get the ends trimmed, but I don't want to lose any length cause I'm trying to grow it back out.

Will the salon or whatever be able to fix it so it isn't poofy every day? Last time I got my hair done professionally it was really niceand smooth.. for about three days.

Also I've been considering an undercut I think its called? Getting the underneath shaved so it's not quite as thick because it's a mess that falls out by the handfuls. But once that starts growing back out or when I'm tired of shaving it, will I have to cut the rest of it short to match the length, or will it look funny if the underneath is growing out and the top layers are a lot longer? Lol