Hi Alice
I'm sorry to hear you're struggling. Replaying things in your head is a form of mental check. Do you know why you check things? Is it to make sure of something, or to just dissipate a feeling of uneasiness? It's really easy to get stuck in a cycle because people often use feelings to judge when to stop doing something, but with OCD your feelings get all out of whack.
The recommended therapy for OCD is Cognitive Behavioural Therapy (CBT) and you can be referred to it via your gp, or in England you can self-refer. There is information on the OCD-UK website here,
https://www.ocduk.org/overcoming-ocd/accessing-ocd-treatment/
Since there will likely be a waiting time I highly recommend the self-help book Break free from OCD as a good place to start tackling OCD. It will take you through what to expect and do in therapy and is written by specialists in the treatment of OCD.
There are also lots of knowledgeable and friendly people here who understand and can offer you support