I'm in the same boat. It comes from too much self-editing and holding yourself up to extremely high expectations. All writers suffer from self-editing to some degree but it's really difficult for those of us just starting out.
Maybe my solution will help you. I try to have as much thought out as possible beforehand. I like to know my protagonist, antagonist, window characters for each, the setting, and a basic plot. The easiest way to generate these is to start with your general idea (or pick a random word in the dictionary if you don't have any ideas) and make word association lists.
For example, let's say I wanted to write about greed. I'd write the word greed and list as many words as came to mind associated with greed. Let's say Money, banker, corporation, food, hugging, vault, and on and on. It doesn't have to be related - just whatever comes to mind. Then I'll pick one of the words for setting. Let's say corporation. Then I'll list out all the people you might find in a corporation: CEO, secretary, technician, whatever. From here, i'll get a protagonist and an antagonist and work from there to get a basic idea of a story plot. Hopefully, the plot will be related back to greed, my original subject.
I really like this way to generate because it's fast and easy. If I get stuck, I can make more lists or ask questions about why i'm stuck on a separate document and try and answer them with whatever comes to mind.
I hope this helps anyone in the same bind.