I appreciate the idea that you’d read a popular book just because I made you want to read its despised sequel.

The real explanation for its existence is probably that Haldeman wrote Forever Peace, which is not a sequel to The Forever War but is a thematic follow up, and then it was a big hit, so he turned whatever other unsaleable idea he had lying around into a “real” sequel.
For me, the magic of Forever Free was mostly due to the fact that I went in cold, only knowing that everyone hated it, and for the first half I was thinking “this isn’t that bad…” until it went off the rails. But if you want to have it spoiled for you, I think this review contains a reasonable summary

https://www.kirkusreviews.com/book-reviews/joe-haldeman/forever-free/