Your's is a common complaint - it's called "getting treed", as hound dogs would tree a coon.
As pleasant as the mamasan is, and as pleasant as the lady is, and regardless of how much the lady says (or shows you) she likes you, we nightcrawlers tend to forget that the ladies are, in the final analysis, following the money. Often the girls you regularly buy drinks for, or barfine, will appear quite 'jealous' if you talk with another girl. However the girl is not 'jealous' in the true sense of the word; she is displeased that another girl is cutting into her customer stream (cutting into her income). There can also be a 'loss of face' to the girl when another girl gets the attentions of "her" regular customer, and this can cause friction amongst the staff of a bar.
Nevertheless, you would be perfectly within your rights, and it wouldn't be a breach of etiquette if you just explained your revised/ expanded intentions to the girl outright. Business is, after all, business. I know a lot of guys who avoid being treed by this method.
But usually people take other ways out of the predicament. One (to start the process in motion) might go into the bar and not buy anyone a drink - just stay a minute, and leave, saying you have to meet someone in another bar. And/ or buy a drink for her and her girlfriend the first visit, and just for her girlfriend next time. Or if she's on stage when you come in, buy a drink for another girl that interests you, and perhaps one for her when she finishes her 'floor' and comes to join you. Or any similar variation on the theme. Some guys have gone to the other extreme, and brought another girl to the bar with them, (that they have barfined from another bar) - not too subtle.
However, in the final analysis, you would be the best person to decide how to wean the 'regular' girl out of your equation - based on how the relationship has developed (or not developed) to date. It seems that there is always a way to accomplish this in a gentlemanly fashion (to best allow her to save face). ---Nevertheless, regardless of how 'gentlemanly' you handle it, there will -rarely- be an unladylike response. -Comes with the territory.
In your case, you also have the mamasan in the equation. And mamasans in the equation can be good at first - they can expedite an intro to the lady you are interested in, and save you from being treed by the real dogs. However it is not usually a good idea to keep the mamasan in the equation past that point, unless you happen to like her as a person anyway. To be sure, the mamasan is quite happy to remain in the equation - as she will get the occasional drink from you (for example), and the bargirl is almost always obliged to her for acting as the original intermediary, and will take care of the mamasan with a tip (especially if the intro leads to a buy-out).
Lastly, on a related topic, a large percentage of nightcrawlers just are not good at saying 'no'. It serves one well to have a large reserve of different ways to politely, or jokingly, say 'no' during an evening's outings.
(Note: this is a 'composite' answer to several similar questions.)