How am I supposed to mix Quikrete to seal up a 3/4" gap in my concrete slab?

You went from way too dry to way too wet. After mixing it according to the instructions on the bag and double checking that you actually have the correct proportions, then if it is too dry, add water no more than 1/4 cup at a time until you get something a little wetter than lumpy oatmeal. You should be able to take a handful of the mix and squeeze it and it will stick together but there will be NO water running out. If you have an old bag that has been setting around for several months, throw it out and get a fresh one. You can not break up lumps in the bag and get it to work.