(A) You can make a short comic from the long story. Similar to Lensing's suggestion, you can just make a comic for one scene or even just one moment. It can help you practice story-telling with comics -- whether it is just a drawing, a few simple panels or pages. It just needs to complete one of your ideas.
Like for example: long story short, this character likes spicy food. There might be a murder mystery going on, but this short comic is focusing mostly on "this character likes spicy food".

(B) If you want to turn your long story into a shorter one, it may be better to think of it as an adaptation. There are likely stuff you will have to change/adapt to make the story work because of parts you cut out. For example, characters might be combined so a smaller cast does same number of plot-tasks or just showing the afterwards of a deleted-scene with enough clues for the audience to figure out what happened.
You can change anything or keep them the same.
( It is your story to make anything happen!
)

(C) If you are looking for very-specific suggestions on how to make your current comic-story more simple & short, you can try posting your what you have drafted to the Review/Feedback category in this forum (Tapas) and asking for feedback there -- well, unless you are not comfortable with feedback from online platforms, then uh . . . nevermind. 
(D) You could check your local library and see if any book for making comics (story) might help you. If you would like to read a particular book that is not available at your local library, check if they may borrow the book from another library for you to request. I wandered in my local library and found "How to Be A Manga Artist" by Balthazar Pagani and Asuka Ozumi; and took notes I found helpful for me -- shared below.
Yonkoma - Tell short story in 4 comic panels. Uses the kishoutenketsu story structure. Can make it bigger by adding more "build up" inbetween panels.
