Yip that is exactly why your rankings are plummeting. Its important to always do an SEO migration if your URL structure is going to be changing.
I would recommend that you have a 301 redirect added from the old URL to the new one. If your website has been ranking well and attracting organic traffic for a long time, hopefully you have Google Analytics installed. Then I would suggest having a look at your top organic landing pages for at least the past year and ensuring that you setup 301 redirects from the old to new pages. Thats if your old pages are still indexed in Google.
If you had Google Search console linked then you also need to create a new profile as I see you now have https setup.
Anyway I hope some of this info helps and you manage to salvage some organic value and rankings.