Compose Your Own Music
Creating original melodies and chord progressions is a great way to improve as a jazz improviser.
You don't have to write everything down, simply invent some new things... a little everyday.
See this great starting point for composition :
Compose-Music.com/Some people call the composition process "improvisation slowed down". We could also say that improvisation is "sped up composition".
When you feel ready, write entire songs/tunes with forms you can improvise one. Write legibly and bring a chart to jam and sessions.
Play your tunes a lot with other people, you will learn a great deal about yourself!