Maybe I'm way off here but I feel like being addicted to kratom probably wouldn't be looked down upon in the same way as, say, being addicted to IV heroin. Depending on how you frame it, especially. You were taking this herb that's widely used in certain cultures for health reasons, didn't realise how addictive it was, and accidentally found yourself taking it just a little too often because you're a busy guy, have stresses in your life, as we all do, and it helped...

I mean that might be bending the truth and I don't know your exact situation but I would think that probably this is not even a lie.
Also... I really, quite strongly disagree with some of the comments above that it's either a big thing, or that it's somehow a betrayal to not come clean immediately.
Is it
really that big a thing? I think when we attach these charged labels to things, or people, like "Addiction" or "Addict" then immediately it makes it into something bigger than it really is. I mean you (and everybody else!) are obviously
a lot more than your addiction - the fact that you use kratom too often is just something in your life that you're dealing with right now, and you haven't told her yet because you're worried about losing her and maybe a bit embarrassed about the situation you've got yourself into. Both of which, by the way, are absolutely fine reasons, and I think anyone with any shred of empathy should understand this.
I'm not saying it's not a quandary of sorts, but I don't think it's in any way so cut and dry as to say that it's such a "big thing" that it's a "betrayal". You should come clean at some point, and you shouldn't wait
too long, but say it wasn't drugs - say it was gambling. You're functional by the sounds of it, so say you're a functional gambling addict but you often lose a lot of money in casinos or whatever. Again, you're a
functional addict, you're not drowning in debt and you are dealing with the problem. Is there a moral imperative that one must admit this to a potential partner at the earliest possible opportunity, knowing that they might have little understanding of it and it might scare them off? Other than the subject of the addiction, these situations are relatively equivalent and your reasons for not coming clean immediately are the same - you want to present your best side because you want to impress her, and this is something you're embarrassed about and you don't want to scare her off.
I would say in fact, that most people would find this analogy easier to accept, and at the same time less of a big deal, and less important to mention.
We could analyse that comparison further and I would quite like to but so as not to bore everyone too much - to summarise, I think that even amongst the drug using community there is sometimes an unconscious bias surrounding certain issues, such as addiction - just because drugs are involved. In the wider drug naive general populace that bias is even stronger, which is why questions like that posed in this thread need to be handled with some delicacy.