What makes us feel replaceable? Research by New York University (NYU) has put it down to self-esteem. If you have low self-esteem in general and feel like you can be replaced, it's a sort of confirmation bias – you may take as truth.
If you have higher self-esteem yet still feel replaceable, it's more likely that you'll work harder so you can become irreplaceable, rather than accepting your position and not try to change it.
In my experience, feeling replaceable doesn't come with sound empirical evidence. The lack of facts to back up the feeling need to be realised. For example, I never genuinely thought my value (in that relationship mentioned earlier) was purely superficial or transactional.
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I had no reason to think I wasn't good enough, or that someone could come in and fill my shoes because I contributed so little. The reality was, I was dating an egomaniac who wanted to make my value feel "less than".
This concept of less than is something others use on us. We also use on ourselves. Why does it have power? Because negativity sticks.
Take the way trolls will cyberbully a person. It doesn't matter how many positive comments they receive online, it's that negative one that will be held onto. The same goes for work or home. Your numerous achievements and contributions in the day-to-day can mean nothing to some people when faced with one harsh critique.
What results from such interactions? You feel like you don't matter enough to warrant being there, and that someone else might as well take over. You feel less than you really are because your value assessment of yourself gets thrown off kilter.
As noted, this feeling can also be influenced by someone explicitly telling you you're replaceable (which is nothing but a power play to make you feel indebted), and can also result from unfairly comparing yourself to others around you.
I see understanding – and thus, working to solve – one's feeling of replaceability as a simple exercise. You must figure out if it's about you, or about someone else.
That NYU study hits on a point I don't agree is entirely accurate. People with who feel replaceable don't just need to "work harder" to make themselves irreplaceable. Instead, they must try to gauge how much of their feeling is based on their own internal standards for themselves. Then, they should think about how much of their purported replaceability is based on how others make them feel.
When your feeling of being replaceable falls into the former category, it's up to you to do something about it. You must find a better way to see your own value and not give negative experiences such influence. This is done by not letting one negative comment undo all the positive ones, or one criticism be confused with overall performance or contributions.
There are a host if advice columns on how to address your sense of self, but when it comes to that sense of replaceability, just acknowledging and validating feelings can go a long way in helping.
When that feeling of replaceability is based more on the way another person makes you feel, i.e. you get explicitly told, or you get a sense of it based on being dismissed, ignored, spoken over, not being replied to, etc., you have options. You can make a choice to stop caring, or you can leave. Break up with someone, end a friendship, or withdraw your labour.
Personally, I'm more of fan of leaving, to be honest. You can't change the way people treat you. But you can change the way you treat yourself.