Hello Gill,
I tend not to deal in labels like “arthritis” as it etches a certain thing on the consciousness of the person. In other words, whether you’re right or you’re wrong, you’re right. Or “what the mind conceives and believes it achieves”. Consider not believing it’s this or that and merely addressing the symptoms.
Relative to diet we would have to probe into the nuances to determine change. Some things are to be avoided and some things are to be consumed. For example you do not mention sources of protein or types of vegetables (other than a reference to “salad”). So it’s difficult to assist you in refining that part of the yoga.
Additionally there are some other nutritional aspects which should be conveyed by your teacher.
Physically, traction in the joints during practice may be helpful. Unfortunately it is only a small percentage of yoga teachers who are properly trained in such things. Joint pain often mandates a modified asana practice away from that which is yang and potentially aggravating toward that which is yin and calming (though this is not at all a reference to “yin” yoga which is something else altogether).
The meditation should be addressing the students blockages or unwillingness to change and move forward in their lives as the joints represent that forward progress. Again, this is something the teacher should be sharing with an interested, dedicated, and committed student - you in this case.
I suggest locating a well-trained, therapeutically oriented yoga teacher, spending some time with that person, and developing a custom tailored practice that will allow you to bathe in the joy of yoga without triggering flares in your physical body.