I had horrible mouthsores when on chemo in hospital. I used baking soda mixed in warm water as a mouthwash. My doctor ended up prescribing a medicine to apply directly to the sores with a swab, but I think the baking soda worked better.

I found baking soda for tooth paste helped along with a water pick and diluted mouthwash helped. Weirdly, drinking Ensure helped a lot. It helped to keep my weight up and coated my mouth. Any tooth paste with mint caused blisters. Hang in there you get used to it.