What was the final straw ?

[Cleaningjusttocleanagain]

This isn’t easy for me, I really don’t know what to do.

Been with DP 5 years , engaged to be married , and one 3 year old ds.

We rent at the moment , he’s always been a scruffy person , and quite frankly can often live like a pig. But he’s a nice man, gentle and kind, a great Dad.

But he can be lazy, unclean, untidy, and unpleasant to live with.

He’s 10 years older than me, and I sometimes feel like I’m living with another child.

I’m at the end of my tether , after numerous chats to him, calm ones and angry ones , nothing changes. I don’t want to hug him, he irritates me , he frustrates me , I’m living a life of misery, I don’t know what to do, or how to do it anymore.

You know what to do. You need to end it. This relationship is over already.

You need to decide whether you want to stay with him or not.
If not there’s things you need to do.

Hmmm

I think maybe the relationship has come to it's natural end?

He can still be a good dad without being your husband

All those things are final straws
You know what you have to do

I doesn't need to be all bad to want to leave a relationship. And just because it was good once doesn't mean you have to stay waiting/hoping for it to get like that again.
Equally just because it's run it's course for you now doesn't mean it wasn't good once.
Its not always that back & white.
But to answer your question, the final straw for me was when I was telling him something which was worrying me and he walked out the room, the house actually, to put some recycling in the bin because it was important - apparently because he could still hear me (yet didn't bother to engage with me once he came back in the house) that was ok.
Except it really really wasn't ok for me.

Make the choice which is right for you.

You can’t fix it single-handedly.
He’s unwilling to try to fix it.
There’s no way forward in the relationship, but there is a way out of it.

If you are merely staying now for the sake of your son, he won't say "thanks mum" to you for doing that to him.

What do you want to teach him about relationships and what is he learning here?.

What do you get out of this relationship now?. Nothing but misery from what you are writing. If you marry this man you will end up fighting him in the divorce courts, is that a scenario you want?.

Re your comment:-
" But he’s a nice man, gentle and kind, a great Dad.
But he can be lazy, unclean, untidy, and unpleasant to live with".

Do you not see how contradictory these two sentences are?. And no he is not a great dad if he treats you and in turn his child like you are being. Women in poor relationships also write the good dad comment or versions of it when they also can think of nothing else positive to write about their man.