Helldivers 2 is the hardest game I've ever played... WTF is even happening?!?

I'm a veteran of vidya games going back to the 80s. I love 3rd person shooters like Ghost Recon, RDR, GTA, Mass Effect, Just Cause, Gears of War, etc. But I cannot understand how to effectively play HD2.

As soon as I drop into a mission and take one step into the red zone, I am instantly swarmed by mutated zombies. I can see a monolith that I'm supposed to destroy, but what the ever loving cripes is a "hellbomb?" It's not in my strategems. I can't equip it from my inventory.

I'm level 2 and I'm pretty sure I'm picking level 2 missions. But this is insane. Enemies are coming from every direction. It's impossible to keep them at bay with the guns I've tried. I can't outrun them. I can't dodge their attacks.

Can I get some tips here please? I must be doing something wrong. Because I swear this is the hardest game I've ever played.

Edit: I should probably also point out that I'm primarily a solo player. The matchmaking I've tried is really wonky where people are constantly flying around (literally) and leaving me behind. I guess they're completing objectives? Then I suddenly appear back in the spaceship after killing exactly zero enemies.