If you really think about it, isn't everything you do ultimately for self gain?
With that in mind lets back track for a second...
From my understanding of light/darkworkers:
-Lightworkers = people who do things to create/help/heal etc.
-Darkworkers = people who do things primarily for personal gain.
In regards to these definitions, it's not necessarily what you do, but really your intention behind your action that defines it as light/dark.
Now that this is established... I would argue that you cannot, no matter how hard you try, do something that is not in your best interest. Or at least perceived by you to be in your best interest.
Follow my logic for a second...
If you think you are a light worker, why do you create? What kind of feelings do you associate with creating? Do you feel good or bad? I would wager that you feel good about giving. SO you are probably creating to feel good, which is ultimately your personal gain, and in your best interest.
The same is true for giving. Why do you give? Because you feel good about giving to others? You feel charitable, or noble, or simply happy that you were able to help someone out in need? All those feelings are positive, thus for your personal gain, and motivated ultimately, if not unconsciously, by the "dark" side of accumulating positive feelings for self serving goals.
Even if you give something to someone and you feel BAD about it, you are sill most likely choosing between the lesser of two evils. For example, giving your significant other a present on Valentines day, maybe you don't really want to do that, and you are frustrated that you have to, but you do it anyway just to avoid the alternate pain of getting chewed out, or by them being hurt by not receiving a present. So basically, you are still giving to avoid hurt, and thus gaining more pleasure than the alternative, which is ultimately again motivated by self preservation and self serving motives (the dark side.)
Now for an even stranger example...
Try punching yourself in the face! (not really...) but maybe think about what would happen
Now that’s a completely strange situation, but it's completely not self serving right? In fact its downright self destructive! Right???
Well I bet you can't punch yourself in the face without it being self serving in some way. Either by proving a point to yourself "Hey! I can too punch myself in the face! I CAN do something that is not self serving!" But even in that fact you are benefiting by proving something to yourself, therefore once again a "dark" and self serving motive.
I don't think there is ONE example in which someone actually does something destructive on purpose to themselves. I would contend that every single action you have ever done in your entire life was self serving in some way and that is the ONLY reason why you did it.
You were either avoiding pain, or gaining pleasure, EVEN if you were, giving/creating/healing or anything else that is commonly associated with the "light" side.
There is no way to selflessly give, create, or heal.
You are always doing it, even at an unconscious level to ultimately receive something of benefit to you.
I think, even if you are acting as a "lightworker" your ultimate root is from the "darkworker" realm and I don't think there is a thing that we can do about it.
I dare you to try!