How can you learn the soft skill of having a backbone? Its something that I lack but it seems innate in my personality. I have no idea how to even begin to change it.
Because you are not used to feeling uncomfortable, you avoid it. All change requires you to face your fears and just do it despite feeling uncomfortable.
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.
Start by noticing when it could be useful. You’ll (probably) find that there are plenty of low-key situations where you can assert yourself without being a jerk, and you can use them as practice.
Be polite, be kind, explain in a manner the consumer can understand - if you can't explain at their level, maybe you don't understand it well enough yourself.