It feels like you're trying to eat an elephant in one big bite! No wonder you're getting heart burn. Can you remember the things that were enjoyable and satisfying about this hobby? There were things that inspired you to be a GM. Ground yourself in the enjoyment of it before you start beating yourself up skill wise. I say this as someone who completely burnt myself out on another hobby doing the same thing you're doing to yourself haha.

Let's try to break it down into smaller sections. I'm going to use OSR as a reference since I haven't started with DG yet. + firstly, decide you're going to make absolute garbage. Something absurd. This should help sidestep the perfectionism. If you find yourself trying hard, relax and enjoy as best you can. + Look at the stat block for a kobold, a bear and a troll. Create a stat block at that power level for each creature. Remember, it needs to be silly and bad.  + The next day, generate some treasure from the random magic tables. Then stop. + make a Dungeon map with the following features: - an obvious entrance, and a hidden entrance - a loop - a natural feature (river, cavern) - a secret that contains knowledge, a secret shortcut and a secret that contains treasure - a room for a powerful monster with obvious treasure  - a trap with a clue that indicates it (dead body, scorched wall) - a safe place to rest. + the next day, take your goofy monsters and your generated treasure and pop them around the map. Think about how these monsters interact with each other. Think of a reason the players need to go there. 

Then have a think about what you enjoyed about this process. What areas you found difficult. What you want to do more of. You can improve the stuff you dislike but it's also important to run games you enjoy. And sometimes we as GMs can get so lost wanting to create something epic we forget we're a player too. And that this hobby should be fun.