People who has “step mums” - did you see them as equal to your own mum?)if she was present and a good mum)

I was very lucky because I have three parents. Two are biological and one is my stepmother, but all three have loved and cared for and brought me up and done the hard stuff, and they are all grandparents to my children. I consider myself fortunate to have had so much love.

This

Tbh, this isn't always true. Thinking particularly of my own stepdaughter's 'mother' who spent most of her time passed out drunk instead of bothering with her kid. She's dead now - from the drink.

Anyway... OP. That's not you. I think you are worrying unnecessarily. Hopefully if your DC get a stepmum, they'll have a good relationship, but she isn't going to 'replace' you.

My daughter has a step-mum, she's 15. I am a step mum to 2 teen boys. Each of the kids loves their step-mum, appreciates them but we are not mum. I love the boys but their mum is their mum. I'm not trying to replace her. On the flip side I appreciated my daughter's step-mum being a good person to her. I would embrace anyone who wants to love & care for your children - it takes a village!

I had the best mum, and I had my dad's wife.

Calm down... my ds was 2 when I split with his dad, by the time he was 3 my ex H had a new wife and he was going all out introducing this lady as 'mum' and her daughters as new 'sisters' to ds....
Ds is now 23... he has absolutely nothing to do with his dad or this lady (totally his choice unfortunately his dad let him down very badly when it really mattered)
Was it hard watching another woman 'parent' my child - yes, particularly as she seemed to think she was some kind of 'expert' and my ex was a truly inadequate parent.
Did I just ignore it unless there was an actual safeguarding concern? Yes
Who do you think my ds turned to at all the important stages in his life? It certainly wasn't her!
Try not to overthink these things, if its a positive relationship, then it's an additional adult who cares about him. If it becomes negative, that's when you take steps to sort the problems out.

I don’t see my stepmum as equal to my mum, no. They’re both completely different relationships. My mum is my mum. My stepmum is my stepmum. My mum loves me unconditionally. My stepmum likes (not loves) me conditionally.

Even nice step mums are more like an aunt.

Your son already has a loving mother, no-one is going to replace you. If he does have a stepmum at some point in the future, you should hope that she is kind and loving and that he has a good relationship with her and that she will take care of him when he's with her and his dad. We all have the capacity to love multiple people. If you have another child, it doesn't mean you'd love your son any less, does it? Kindly, I think you need to address your insecurity through counselling.

You've already decided this woman is going to either steal your son or hate him and you're stressing about it.

The reality is, when she does exist, she will be kind to him, and they will probably like each other, but you'll always be his Mum, the one who loves him unconditionally.

I met my stepmother when I was a toddler and she has never once, not for even a moment, eclipsed my mum in my affections. And my mum isn't perfect but she's the only mum I'll ever have and she tries her best.

How sad that your first thoughts are about your position rather than your child’s happiness.

A step parent is a bonus not a replacement. Stop thinking of love as a finite portion and start thinking of it as something that has unlimited boundaries and you’ll be a lot happier. Don’t store up trouble where it’s not.

I have a lovely stepmum. I love her but she's not my mum. It's a completely different relationship and no one could ever take my mum's place.

My parents separated when I was young. Both had several partners before finding their soulmates.

Very fond of various step parents, entirely neutral about others and deeply disliked one.

But my mum and dad were my mum and dad.

I am a stepmum. In no way am I a second mum. It's a different role.

I say this with kindness op, I really think counselling would benefit you.

You have already projected a huge amount of angst onto this woman who doesn't exist yet.

It's very hard having a rough childhood and a crap Mum, I've been there, but you can't let that destroy your relationship with your children.

All you need to concentrate on right now is being a great Mum, and it's obvious how much you adore your son. Don't catastrophise about the future too much, concentrate on the now.

Your ex husband has likely had the same thoughts about you meeting another man. As long as you both have a good co-parenting relationship, none of this should be an issue.
(also - cross that bridge when you come to it, but it hasn’t even been built yet!)

I’ve had two stepmothers - once as a teenager and once as an adult - neither of them were my mum, they were my dad’s wives

A good relationship with a stepmum is addition not replacement. Additional adults in your child's life who listen to them, help them out, is usually good thing.