AIBU to believe a Solicitor acting for the other party can't make false accusations against me?

A solicitor will write what your ex tells them, since he’s paying them. My solicitor used to cringe at some of the utter garbage that my ex said.
You need nerves of steel or representation.
Write a response to all of his accusations with evidence to support you if possible.
It's often men following ‘the script’ and judges see straight through it. My ex swore at the judge and stormed out once.

My ex quickly worked out he could make up any old shit and no one stopped him. Got him nowhere. His tactics were obvious.

but it resulted in a whole world of stress and pain for me along the way, worrying the court would believe him (where they just want you both to act like adults, stop fighting and do the right thing by your child)

I’m a solicitor. Your solicitor is obliged to act on their client’s instructions. They also have a duty not to mislead the tribunal, but it’s only misleading if they advance a point they KNOW to be untrue. They do not have to KNOW their client’s instructions are true, only that the client says they are true. If the client says “I want you to say that Apricottoast attacked me, even though that didn’t happen”, they couldn’t say that. If he told them to say you attacked him, and he maintained that that did happen, they would have to act on his instructions.

I hope that helps.

They’re acting on behalf of their client. Its horrendous and I sympathise with you - I’ve been cross examined by a barrister as part of a CSA trial (I was the victim) and it’s really upsetting.

But they’re only doing their job.

I wouldn’t say they take it as truth they just present it as such.

They do have a code of conduct and a regulator - you could make a complaint but chances are she is legitimately presenting what your ex has said about you. You simply need to counter this in your witness statement and remain steadily child focused - which btw courts prefer.

My ex's solicitor is making all kinds of ridiculous statements about me. I assume they are all coming from my ex, who is behaving like a complete twat. However, I have gone through each one of them with my own solicitor and she's countered them all.

Get a solicitor. If you are on a low income, is there no financial support available in the UK? (We are abroad and I could apply for support but I'll be back at work soon -currently on maternity leave - and would earn above the threshold.)

Actually it's the opposite of what you think. They are there to argue whatever their clients want them to unless they know for certain that it's a lie.
The judge will be well aware of the sorry of tactics, stay calm and try to remain dignified and remember in a cms claim tribunal the judges have hurd and seen it all before and the fact that your ex can pay for a solicitor (whilst claiming poverty) and you're self representing says everything about the sort of man he is.
Also when your ex only has mud slinging on his side whilst you have actual evidence means that they have no defence.
My ex and his solicitor made all sorts of nasty allegations about me to the judge, the judge still ordered a lean on his house so that when he sells it I get my £s back cms he owed (he had fiddled his earnings for over a decade until the tax man caught him🤣🤣🤣🤣🤣).

Solicitor acts on good faith, they take what their client tells them as the truth.

As an example, he could say you were drinking a bottle of vodka a week in the morning while driving the children to school. You can say you were drinking water. Witnesses saw you drink from a water bottle. You can't prove it wasn't vodka when he has said he saw you put vodka in the bottle. Proving a negative is very challenging.

Their job is to win in their clients favour, not yours.

They’re acting on instructions and the representations made by their client to them, presenting their clients case. Court is supposed to be the venue to test the evidence. Solicitors can’t be expected to intuitively know what’s factual and what’s not.

No, they aren't there to interrogate the truth of what their client tells them. It's in court where you can prove his claims false.

A solicitor acting for she opposition party is supposed to make you look bad and disparage you. You need nerves of steel and a good support network.