Plugin updates and old translations

Hi,

I’m getting a bit tired here. I don’t understand what I’m supposed to do.
So, I have this plugin with almost 2200 lines. I translated them all with PoEdit (lot of work!!).
Everytime the plugin is updated, I lose my translation. I tried to put my translation in loco>plugins, and in languages>plugins, still the same.
Problem is, when the plugin is updated, they add new lines and delete some others.

How can I load my translation from their new file, add/delete/etc not losing what I aldready translated?
I tried adding a new package with my translation, it works, but when I synchronize to get new lines/delete/etc, everything just disappears and I’m back to 50% translated when it should be 95%.

I don’t know if I’m being clear here, english is not my first language and I’m a bit annoyed by all of this.

Thank you for all the help you can provide me!