Hi everybody, any help is very appreciated. I don't want my plants to die like they did last year :(

I've never used DE against slugs/snails, but I do know that Sluggo or some generic equivalent works very well (just sprinkle it around the base of your affected plants, you don't have to do the whole garden). It could also be flea beetle damage, which usually isn't fatal and should end when it starts really warming up, or earwigs (which I have no idea how to eradicate). Usually, I'll just take care of the snails - which are my worst killers of small seedlings - and just leave everything else alone. Earwigs/flea beetles won't kill anything except very small seedlings, and they just leave you with some ugly plants until they grow out of it, snails/slugs are a different story.

*edit* you might also cross-post on thevegetables forum, you might get more suggestions there, too. I'd suggest saying something about where you garden either in your post or changing your profile to reflect it, so people can give more accurate advice on that forum.

Grasshoppers?

leaf borers or flea beetles. DE works pretty good on flea beetles though. Leaf Borers is what I usually see on beets and chard and spinach and lettuce. I've never grown corn and powdery mildew is my usual squash problem.