Not banned forever though, at least in California, it’s a five year ban following a 5150. Also, the ban doesn’t apply if the hold was observational or voluntary, i.e. a psychiatrist decided that you needed to be held in a facility and you went along with that decision, rather than being forced by a court, for example.

The federal form 4473 (which you fill out whenever buying a gun through a dealer) also gives some exceptions. If you search “4473 exception for involuntary commitment” you can find more info.

Depending on the details of the particular situation, you may still be able to buy a firearm legally. It could still be worth pursuing.

Edit: my bad, I read your other comments. There are ways to challenge a ban due to an involuntary commitment, but you gotta hit up a lawyer, etc for that.