I have not gone. But I *have* gone!

I have failed to attend before.

or

I have been absent before.

(Basically, any sentence where you can embed the negativity into the semantics into one of the other words (such as fail or absent) — rather than using not — will eliminate the ambiguity of where not applies.)

This is a tricky area of English grammar. I think we might be missing a tense or something.

Your original phrase I have not gone before... does mean what you want, but it's a variation many people would not pick up on. This is because the not can bind to both have and to gone, but most native speakers will assume it binds to have. To make it bind instead to gone, you have to split the clause.

An experienced, native speaker would probably say I have sometimes not gone before... which naturally conveys the meaning you want. Even if it would sound awkward to some speakers.

You don't have to talk about it in the past tense. You can describe yourself, rather than describing your past actions. The most natural thing for me to say is

I don't always go