Twinkle It doesn't surprise me in the least that you feel propelled back to that dark place, now that you have discovered he was having an affair all along. What's actually going to help you somewhat is if you can get angry about that; angry that you were lied to and that he would have preferred your continuing pain, rather than honesty about his real reasons for leaving.
Men in your H's position would rather prolong your agony than live with the consequences of the truth. They would rather pretend to be a tortured soul wrestling with changed feelings and huge soul-searching, when the reality is that they are a common-or-garden adulterer following the same life script as a million men, the world over.
Your post is also a salutary and apposite reminder to women who are currently hearing the same tired old lines that you heard 2 months ago. Thank goodness you know the truth now. Celebrate that Twinkle, because knowledge is power.
Now back to that script, because there is some good news ahead for you. Looking at your OP, you are only 30. You have one much-loved child. If you've been with your H since you were 16, the world is now open to you with all its opportunities. At some point in the coming weeks and months, that's going to feel exciting.
Statistically, very few relationships that start as an affair go on to be successful. Some do, of course, but IME, only if the couple regret some of their actions (if not the relationship) and try to do right by the people they have left behind, including giving them some closure and honesty.
It is far more common that men in your H's position have confused lust and excitement, with love. The fact that he was also inexperienced when he settled down, lends weight to that. At some point in the future, that "new relationship" phase will end and other competing pressures will start to intrude, such as seeing and maintaining his child, facing up to the people who have in the past, believed his hackneyed tale of lost love, etc.
I bet he has woven an incredible tale to friends and family that he met this woman after he left, or that "nothing happened" with her, before he left you. I hope you will now disabuse them of this notion, if you get the chance, but if it helps, few people with even half a brain would fall for that tale anyway.
Meanwhile you can and should move on now. Use your anger wisely and in ways that will help you, not harm you.