I had bad constipation when I was on Lithium recently. Like you, I ended up going every 7-9 days.

I was also taking fiber supplements and increasing my water intake.

I also tried probiotics. I researched online and found some people taking lots, but I ended up taking 30 billion per day (3 capsules of 10 billion).

You can get some probiotics with 150 billion bacteria in them (I think it's called Ultimate Flora). I can't say it stopped my constipation because I was still going every 7-9 days but I noticed I wasn't having the same difficulty I had before.

Probiotics can take a while to start working, so you might have to be patient.

I recently stopped Lithium and the problem went away completely.