Google Search Console not indexed pages

I have an issue in search console which lists many pages as not indexed. This was caused by loading a sitemap which had the incorrect url structure. Google ended up indexing all the pages before i realised the url structure was wrong. I subsequently loaded a new sitemap with the correct url structure and all pages are indexed. When i go to search console it is still showing index error on the old urls. Its been almost 1 year like this and i do not know how to fix this. I tried adding the not indexed urls to the robots file but that did not work. I think this is affecting my SERPs position. In 1 year i have seen no improvement on SERPS even though i have built up 100+ do follow backlinks from DA 50+ sites. I need to fix this issue ASAP, any ideas ? I was thinking to take the extreme option of removing the domain property from search console then adding it back. Is there any drawbacks of doing this ? Like losing my backlinks i’ve built up over the last 1 year ? Google penalising as presumably it will treat is as a new website ? Anything else i need to be aware of ? Thanks in advance to anyone that replies.