I'm so bloody fed up with having a stroppy 5 year old.

[throughgrittedteeth]

Just that really. I fed up with the tantrums and arguing and rudeness.
DS is 5 and is just horrible to me. His dad and I aren't together but get on very well and he doesn't behave like this with him. Ex came in last night when he dropped him off and the change is DS was immediate, ex was shocked and sat and had a stern chat with DS about how he spoke to me and his behaviour. It's great to feel supported but it's so exhausting when it feels like your DC just doesn't like you.
Ex and I have always parented very similarly with similar sanctions for bad behaviour, it just doesn't seem to work for me anymore. I know there has been a lot of change for him since we split but it's been 18 months now since we moved and I can't keep excusing his behaviour because of that.
Please help me, what can I do?

I know your DS is only 5 but it may be worth attempting a "grown up chat" with him. It could be that, as boys do, he idolises and looks up to his dad and is angry with you for not being together anymore (definitely only suggesting, this is by no means true and by no means even if it was would it be your fault that you are no longer together).

Rather than a stern chat, maybe sit him down and explain that mommy would like to have a grown up chat with him now that he's a bigger boy. Explain that you've noticed he behaves much more like a good boy for dad and that it makes you so happy to see him being a good boy for his daddy. Then explain that you've noticed he doesn't use his good boy behaviour at home with mommy and that really makes mommy sad. Ask him to use his words to tell you if there is any reason why he's not always a good boy for mommy.

It may work, it may not but in my experience, kids can be fiercely independent and crave the feeling that they're being treated like an adult and that what they say and feel is valuable. Hopefully that approach works for you