I'm another one who would deter you from going to Relate or going for couples counselling at all, at this early stage.
You are still in a state of shock.
I hope your husband communicates the right message to the OW. Not just the words he uses, but that how he looks and sounds is congruent with those words.
I'd also advise you to be a little more circumspect about your husband's apparent remorse and sorrow.
I agree that it's a normal human experience for him not to have understood the true enormity of his actions until he'd seen the effect of them on you. This must not be discounted or trivialised. We have no trouble understanding this concept when it applies to criminals feeling extra remorse when they see the impact on their victims and their families and this is no different.
However it is also normal at this stage for your husband to regret you finding out about the affair - both for his own sake and yours.
It would be revealing for you to find out how much he regretted what he'd done and how much guilt he felt the day before discovery. If you have any correspondence written by him just before discovery that supports anything he is saying about feeling remorse before being found out, so much the better and therefore more likely it is that his remorse is completely genuine.
If you don't or if he has deleted it, I hope he is honest about this aspect. I also hope that you have enough additional evidence than just his word for it. For example, if he was trying to cool things off with the OW, you would expect to find fewer texts and phone calls to her on his bill.