Have you thought about how you want to tell him? If you find peace in having a clear plan, it might help to treat it like a project — step by step, with timings, contingencies etc.
For example, if you file online the fee is £593, which you pay upfront. He won’t get the notification for about 7–10 days after you submit it. So, if your plan is to file first and then tell him, think about how you’ll pay. If the money comes out of a joint account, will he notice? Do you have a way of covering the cost discreetly?
If you’re worried he might try to talk you out of it, or that you might hesitate, filing first gives you the momentum. You can then choose to tell him immediately, in a few days, or not at all — just let the email land when it lands. I’ve known many women who’ve done exactly that.
Personally, I’d also be ready with a message for the OW, maybe with a happy-family holiday snap or a photo of you two in bed last week etc. Send it right as he gets the divorce notice so that when he runs to her for comfort (even if he doesn’t say why), she will at the very least not quite be the cosy comfort blanket he was hoping for.
On the other hand, if you plan to tell him you want a divorce before filing, be prepared that he might try to spin the story to suit his narrative so to that it’s his decision. He might even cover the cost to make it seem more civilised — and while that may sound fair it’ll be a massive power play from him. That would infuriate me, but maybe you want it to be amicable and mutual. You’d be a more gracious woman than me though honestly if you could do that!
You could also ask your solicitor to file on your behalf if you want to keep things even more contained — just bear in mind it’ll cost more.
Hope you have a good chat with solicitors to find the ones you like - also worth asking your parents for recommendations- you might be surprised who they know, their generation are divorcing like billio so they’ve probably got friends they can ask!