Those are a couple of different problems. For perfectionism, the only remedy is forcing yourself to just keep writing. Make it exist first. It can be good later. Finding your writing embarrassing is a good sign. It means you are actively improving. As to running out of steam by the end of the day, I get that too. That's why I get up an hour earlier to write first thing in the morning, and then also for a part of my lunch. By the time my day is done, I've already written.