I suggest thinking about this in a slightly different way. Don't tell yourself that not having this skill is innate to your personality - that's a belief founded in a fixed mindset. Try to adopt a growth mindset on this issue. You have lots of little opportunities to stand up for yourself, which will over time add up to the ability to stand up for yourself when the stakes are high. I'm working on this issue too, and find the approach to be the most useful.