What Do The Mushrooms Eat?
It's important to remember that fungi are heterotrophes. Like animals, they must get their energy by consuming other organisms. Forests are typically made up of trees, which are autotrophes: they can make their own food from the sun.
Most fungi eat decomposing plant matter. Most of the mushrooms you might be familiar with at the grocery store begin life as a spore that has landed on some dead plant stuff, either a log, a pile of leaves or straw, or else commercial grow media. The spore grows into a network of myclium that consumes the plant matter, and when the fungus has had its fill, it fruits mushrooms which spread more spores and then often dies of starvation, having completely exhausted its food supply. A log that has been eaten by fungus cannot even be used as fuel, it is so thoroughly consumed.
For your mushroom forest, you will need to explain where all of these massive mushrooms are getting their food from. This could be an opportunity to add some additional ecosystem elements to your world! Perhaps some vast horde of migrating animals seasonally comes to this forest to procreate and die, leaving their corpses to be consumed by the mushroom trees. Or maybe they just come here to poop - before carrier pigeons went extinct, they used to blanket [tree-filled] forests with so much shit that you can tell what years they migrated over a particular area just by looking at the growth spurt in tree rings.
Prototaxities: Real World Fungus Forests (maybe)
400 million years ago, there were likely no plants taller than 1 meter. Plants had not yet evolve the ability to make lignin, so they were structurally limited. And yet the fossil record shows evidence of large, trunk-like organisms that grew upwards of 8 meters! These are the prototaxities, and they are thought to have been fungus. It seems they did indeed grow as a sort of forest (sometimes) (maybe). What did they eat? That seems to be a heavily disputed issue. One prominent theory suggests they had a symbiotic relationship with algae, which might have provided photosynthetic energy. However, symbiotic relationships between algae and fungi are usually called "lichen", not "mushroom".
A Final Suggestion
Speaking of millions of years ago, when plants eventually did evolve the ability to make lignin, it sparked a global extinction event. At first, few if any organisms on earth were capable of breaking lignin apart, so as plants died there was no one around to decompose them. These dead plants piled up everywhere, refusing to yield their carbon back to the atmosphere and sparking a horrific climate catastrophe. I imagine that whichever organisms first evolved the ability to break lignin apart must have really gorged themselves though! Maybe this could be the case for your mushroom forest: a mega-huge mushroom species discovered an ancient, buried forest and now feeds on millions of years worth of preserved wood. Just a thought!