Definitions from The Online Plain Text English Dictionary:

Insecure
• (a.) Not effectually guarded, protected, or sustained; unsafe; unstable; exposed to danger or loss.
• (a.) Not secure; not confident of safety or permanence; distrustful; suspicious; apprehensive of danger or loss.

A woman here discussed a situation that makes her extremely uncomfortable and I noticed how she was repeatedly labeled as insecure. It made me wonder when did it stop becoming the partner’s job to make the person they are in a relationship feel safe? There are extremes when a person has low self-esteem and has a neurotic need for constant reassurance. This is unhealthy, and not what I’m talking about. I’m referring to a normal feeling of uneasiness because your partner has an unbreakable bond with a previous lover, and a reasonable desire to be reassured the relationship with you is what he is more concerned about. If a partner can’t provide that, I don’t see why they deserve a new relationship.

If a person who otherwise is self confident, and has healthy interactions with others but doubts the security of their relationship, maybe your partner is doing something wrong. I don’t see why the “insecure” person in this scenario is at fault.

I don’t feel it’s my job to make myself feel comfortable with BM’s existence. I'm not the one who has an Ex that is a constant presence in our lives. How my husband handles BM determines whether I feel comfortable with BM’s existence.

It isn’t always a character flaw on your part to feel insecure. It’s only natural to feel insecure from time to time in the step situation, especially when the partner reinforces your anxieties.
We stepparents beat ourselves up over not feeling and acting how we should. If you are frequently feeling what was described in the definition above, maybe the problem is your partner. Not you.