I think the first step is to look a little bit at what's happened.
Cause 1: losing traffic Cause 2: losing click through rates Cause 3: losing value of the click
Losing traffic is possible across "several" sites, so I'd look at this first. Easiest to see it in your adsense reports are the impressions: if they drop, there's no way around to a drop in revenue. -> solution might be to try to recapture the audience.
Losing Click Through Rate (CTR) is mostly due to the look of the ad/site combination or from getting visitors that changed their ways. Should be easy to see in your adsense reports. -> solution might lie in the layout or design of your website
If the value of the clicks dropped this is where you can do the least. If this dropped by 90%, then I'd suppose I'd be smartpriced - Google thinks their advertisers don't like your site(s). Recovering from this: I guess it's unlikely within the adsense program.
There are other ad networks out there, that might be an option if you still have the traffic and interested audience.
There's one comment of yours:
I'm reaching the point were I don't know if worths keep paying hosting for some of my sites.
which sends some bad vibes. Sure if my income would be slashed by 90%, I'd reconsider some things, but not one of them was made because adsense or any other revenue source existed, so I'd never consider just dropping them either.