It seems to be an unhelpful fact of life that we unconsciously give out non-verbal cues to our personality. These cues are then picked up unconsciously by others (studies have been confirming this phenomenon for decades).
Thus if you are a “giver” you’ll tend to attract “takers.” Speaking as a paid-up member of the “rescuers” club I’ve tended to attract those who need to be rescued. The problem is, being aware of the problem doesn’t help much because we can’t easily turn off or modify those cues we’re radiating all the time. There have been many times in my life when I’ve felt as though I have a flashing blue light on top of my head (the reference is to Doctor Who’s TARDIS, the time-and-space police box by means of which he travels the universe and inevitably ends up helping total strangers, often at great cost to himself).
I’m not sure there’s an answer except to be aware of the phenomenon and be ready to punch the eject button as soon as you see the familiar pattern playing out once more. And maybe stop yourself from those early moves you perform naturally — the moves that confirm those unconscious signals were accurate, and that you will be the giving partner as always. Don’t pick up the check, ask for help early even if you have to fabricate a reason, resist the temptation to step in.