Sorrow - I would echo the Not Just Friends Recommendation - and Mumsnet.
I think recovery depends on so many factors, so it would be helpful to hear more about what happened to you - the story of the affair and the story of your marriage.
One piece of urgent advice I will give you is to give vent to your feelings and questions, but bear in mind that anger is very often met with defensiveness and a lack of complete honesty. So, if you feel the anger bubbling up while he is telling you what happened and why, it is sometimes best to call time at that point and return later.
To your husband, I would say: be honest straight away - don't hide anything, because lies told now and drip fed information will hinder recovery.
One of the things I also want you both to bear in mind is that one cannot forgive until one knows all there is to forgive. Holding back now on details, however small, will harm both of you, although the with-holder often doesn't realise how much this harms them too.
Your quest for knowledge depends so much on the type of person you are. If you're someone like me, you will need to know everything, however painful - and however trivial. If you are like this, you might also want to piece together events like a jigsaw - look at the affair as a timeline and with a diary, work out when the peaks and troughs of the affair were. Review the significance of key dates and events - this helped me enormously.
Later on, I constructed a timeline of when the build up to the affair started. You'll need a good memory for this, but it really helped me make sense of what had been a 10 month build-up, and 4 months of an active affair.
For most of us here, infidelity has been the worst event of our lives - the devastation is enormous and it often comes as a surprise that it has a ripple effect on every aspect of life. If this is a recent discovery, you will be in shock - and reading a bit about post-traumatic shock will help you understand your feelings and why they are happening. In the early days, you may feel unsafe - infidelity really does make us feel threatened in a way that feels surprising - and is very rarely written about.
Depending on the details you give us, I would wait a while before counselling and try to discuss events together for now. Counselling works when both partners are committed to honesty and it can really help if you get stuck on certain issues.
Have a trawl around these boards for more help, but it might help your esteem a little if you read a post I made yesterday on the "In turmoil over my affair" thread. This is not about you, or even your relationship, but it is to do with your husband; the fault is with him. You are not to blame for his affair.
Will say more when you give us more info, but be reassured, it is possible to get through this terrible life event and emerge stronger and happier as a couple. As Grace says, it provides a once-in-a-lifetime opportunity to get the husband and the marriage you deserve, and always wanted.
Although this is painful - and there will be more pain to come - the worst is really over. That period of time when you were actually most vulnerable; the build-up to his affair and the affair itself, has passed now. The time when you were an unknowing victim has gone and your knowledge gives you enormous strength.