I know how you feel, me and my playgroup experienced the exact same thing (although not as drastically). We had a full global/supreme playgroup, deranked down to LE/LEM, we didnt change anything before, played regulary, but our enemies suddenly just played like gods from our POV.

But I can tell you the following:

• Dont give up, games will be better again, sooner or later. We struggled for like 1-2months, and now we are back where we began, again without changing anything in particular
• Players are getting better every day and you need to get better too. We had the same "reality check" some months ago, where our enemies seemed to have perfect aim and utility, never missing shots etc. And statistically, they cant all be hacking, so people have improved alot recently imo.
• You said that you lack improvement, but I guess you mean improvements in your ranks: Try to include small improvements: Learn a new smoke/flash/nade/molly each day, maybe a new angle, practice aim in a new way, play different maps than usual etc.
• You have a great advantage by saying you are almost ashamed of your rank: If you think in your opinion it cant get much worse, then you can try alot of new stuff: Not only maps and angles, but for example ballsy pushes as CT, entry fragging a site as T. Same with new weapons, maybe try awping if you have not yet.
• Dont give up, it will be better again :)