Write some pieces just for fun with no particular outcome in mind or regard for quality until you know what that feels like. You don't have to show them to anyone.
It is very difficult to mix being creator and being critic at the same time. It is best to alternate roles at different times. When I am writing music I just write what comes out and much of it will be rubbish. It doesn't really matter because I can edit it later, throw away the not so good bits and keep the good bits. Then I can have another phase of creating and expanding on the good bits (again many additions might be rubbish). It is a process of evolution - creation/destruction - like a little child building some blocks and then knocking them down.
You may think that a great lyricist such as Leonard Cohen comes out with that stuff immediately but he doesn't. He says he goes through about 10 versions of a song before he is happy with it and can take up to a year and had many on the go at once. Of course the public only see the end result not the process!
Expecting to get it 'right' first time will get you in the habit of blocking your flow.. the imagination doesn't like to be stopped continuously.
There are many other exercises you can do to get your imagination flowing. Here is one called image streaming..
Project Renaissance, Image-Streaming
I played a trick on my girlfriend the other night. I ordered to her "Make up a story". Of course she "blocked" and couldn't come up with one to order. So I said "OK. I will make one up and you try to guess what it is. I will just say Yes, No, maybe, close etc". She relaxed and started saying "Does it have animals in it?, Is it on the moon? Is there a boat that floats from planet to planet, etc.." Anyway, after about 30 questions she had made up a lovely story. I was just saying Yes, No, maybe etc randomly. I had no story actually in mind! A bit naughty but it worked..
Of course, because she wasn't forcing her imagination or felt responsible for the quality she could make up a very lovely story. It wasn't Shakespeare but would have made a nice bed time story for kids.
Reverse the roles, let your imagination control you. Of course, then quality doesn't matter because its not your responsibility! Who are you to be responsible for your imagination anyway? If it was someone else talking rubbish with the occasional gem in you worry about the bad bits or pick out the gems?