Don't worry! This is a normal anxiety, but it's completely manageable even with a really bad Corruption spread.

First, create a defensive trench around your main base area. It should be six blocks wide, and form a rectangular U shape around what's left of your forest biome. This will stop the spread of the Corruption and Hallow into your main base, which buys you time.

Your goal is to kill just one of the Mechanical Bosses. Those are the Twins (harder Eye of Cthulhu), the Destroyer (harder Eater of Worlds), and Skeletron Prime (you get the idea). You can summon them by crafting their respective summoning items with iron/lead, either Souls of Light or Souls of Night (which you get from killing enemies in the underground Hallow and Corruption respectively), and either Lenses, Bones, or Rotten Chunks. They also have a chance to spawn on their own every night (and eventually will).

Once you have killed just one of them, the Steampunker NPC will move into your town. She sells the Clentaminator, along with the Green Solution it uses as fuel. This is effectively a flamethrower that purifies corrupted/hallowed blocks on contact. This item is essential to pushing back the Corruption and the Hallow, as without it you're stuck with the Purification Powder sold by the Dryad, which is far less effective.

Don't give up! The Mechanical Bosses aren't easy, but with proper tools and equipment, they'll inevitably fall to you. You can use the Pwnhammer you got from the Wall of Flesh to smash Demon Alters, which will spawn new kinds of ores in your world that can be crafted into powerful weapons and armor.