Lopez, 52, revealed details of the surprise engagement in Tuesday's "On the JLo" newsletter which was appropriately titled, "How It Went Down." Affleck popped the question on April 2.
"Did you ever imagine your biggest dream could come true?" Lopez began.
"Saturday night while at my favorite place on earth (in the bubble bath), my beautiful love got on one knee and proposed. I was taken totally off guard and just looked in his eyes smiling and crying at the same time trying hard to get my head around the fact that after 20 years this was happening all over again, I was quite literally speechless and he said, 'is that a yes?' I said YES of course that’s a YES."
Lopez says she "was smiling so big and tears were coming down my face" as she felt "so incredibly happy and whole."
"It was nothing fancy at all, but it was the most romantic thing I could've ever imagined...just a quiet Saturday night at home, two people promising to always be there for each other. Two very lucky people. Who got a second chance at true love," she shared.
Affleck first proposed in 2002, but they called off their engagement two years later. This time, instead of a 6.1-carat pink diamond, the Oscar-winning actor popped the question with a green diamond. In Tuesday's newsletter, Lopez shared new photos of the giant sparkler and a throwback shot from the first time they got engaged.
"Green has always been my lucky color and now, for sure, it always will be," Lopez concluded.
As for how Affleck's ex-wife, Jennifer Garner, feels about the proposal, a source tells E! News the actress is content.
"Ben told Jen and the kids, so they knew about it," an insider claimed. (Garner and Affleck share three kids.) As for how Garner supposedly feels about Lopez, "Jen is happy for Ben."
Garner purportedly "knows that J. Lo has been a positive influence in certain ways."
MORE: Jennifer Lopez on her "beautiful love story" with Ben Affleck