Wrong tax credit claim

Hello, I want to ask you for an advice about wrong claim. The thing is that I made a wrong tax credits claim as a single, but i was in a couple. The reason for doing that wrong claim was my misunderstanding of guidance in the form, when I was applying for the claim. I am an EU national,
I ve been over 25, I worked 38 hours a week since the start. My gf was not speaking a good english either and cant find a job because of that for a longer period of time. Also she was 23 years old. I was in misunderstanding and I claimed it as a single. 2017/2018 was the first year.
Now when I received a tax credits renewal pack I noticed, after my language got better that I made a huge mistake and I want to make it right... but I m very scared about the consequences, the penalties and so on... I ve been through tax credits calculator and it told me we can get more as a couple, with considering that we didnt earn anything in UK before(I tried to did it like we would arrive now to see amount I should be given).

I want to say It was not a fraud attempt at all, just my mistake and misunderstanding, which I realized now

So what should I do now people? I want to send renewal but I should write there my change of circumstances like I ve been living with her from beginning of our life in the UK? Is there any option to inform HMRC about the wrong claim and add it to renewal? Like attachment or something?

Thank you for any help regarding this matter.