The above complaint is a very common problem among newbies at improvisation. They don't understand how to keep an improvisation going. Why? Because they have it backwards!
You're not supposed to keep an improvisation going. No. That's the wrong approach to improvisation. The right approach is to let go of your need to control the outcome. Then and only then will your intuition come to your aid.
Listen, you may want to create something beautiful on the piano, but it is exactly your desire that is creating blocks! How? Because the ego is never satisfied and wants a perfect music, a good music, or something that will satisfy it. The ego is insatiable and is never satisfied.
That's why when you focus on and enjoy the process of improvising, you free yourself from this trap.
Many of us are obsessed with creating a product. Something we can be proud of and show off to friends and family. Look, there's nothing wrong with wanting to have something you can call your own, but when you are only concerned with creating the end result, your creative self (inspiration) will dry up and leave you feeling empty.
But, if you put process over product, not only will you enjoy the whole experience more, the outcome will be far superior to any forced ego product you may end up having.
To enjoy the act of making music, we must let up on ourselves and adapt an attitude of exploration. We must allow for mistakes and listen for where the music itself wants to go. This idea may be strange to some of you. I can understand that but once you really let go of trying to control the outcome, wonderful things begin to happen!