I am not an expert, but I have the most experience with bettas out of any fish. I defer to anyone who says they know a specific reason why what I'm saying is inaccurate, HOWEVER, I truly thing you should move your better until you get it under control. Too many things can happen to it while it is weakened from the nitrite situation.
I hope I'm wrong, but I think the conditioner you have might do nothing for nitrates and nitrites. Check the bottle.
I looked it on a retailer site to verify the following about Seachem Prime: "Prime removes chlorine, chloramine, and ammonia. It converts ammonia into a safe, non-toxic form that is readily removed by the tank's biofilter."
There are conditioners that handle all of it. I have a brand called KORDON and it "detoxifies nitrate, nitrite, ammonia, Chlorine & chloramines". I also have a brand that only does chlorine and chloramines. So, I guess conditioners come in all different varieties.
Also, have you cleaned out the extra food waste? Do water change, but also vacuum that substrate and get it out of there or it will continue to produce more nitrates and nitrites as it breaks down.
I do hope this helps. Not trying to be a know-it-all. I certainly don't know it all! But, I've learned a lot of lessons the hard way, so just trying to share tough lessons.