Stupid question about tense in internal monologue

Like, "I wonder what it meant" versus "I wondered what it meant"?

Either one is possible. If you're reporting on what "you" thought back then, the past tense is entirely appropriate. If you want the thoughts to seem present, like dialogue, then put them in italics and word-for-word the way they would have been thought in the first place. Whatever serves the purposes of the passage the best.

If it's internal dialogue like here's an excerpt from mine -
"Oh, that’s fair. At the same time, I was relieved because I didn’t plan to go through that kind of nightmare again." - then it can be present tense.
But I would love to here an expert answer.

I write in first person POV too and I use past tense, except for the internal monologue.
I've been told that the internal monologue should always be in present tense. I don't agree fully though. For example:

What had I done? (can be in past tense)

Why is he looking at me like this? (present tense)

If the thought is narrated though (without the italics which means it's not internal monologue), I would always use past tense. For example: I wondered if I could have done things differently instead of I wonder if I could have done things differently.

If you use present tense for the thoughts, you should consider using the italics, I think. Otherwise, it's jarring and makes it look as if you have mixed tenses in your story.

Italics are pretty standard when reporting thoughts in real time. It's not an absolute rule, I don't think, but most editors seem to tilt that way. But that's not a major issue, and can always be changed according to publishing house preferences.

I don't typically use italics in first person, because, well...first person is already told in their voice. Their thoughts are evident by their way of explanation, I guess?

I don't know. I've never come to a point where I thought, "I definitely need to separate this particular thought with italics." Third person, definitely, but not for me with first person.

Good luck If nothing else, your editor one day will be able to correct it for you.