Antihistamines seem to help some cats even when the problem seems to be psychogenic. There are several anti-anxiety medications which can be helpful and it is often possible to use them for a relatively short duration of time then wean the cat off the medications. Sometimes, just breaking the habitual cycle of licking and chewing helps for a long time. I took her to the vet in January and they ran extensive tests on her and found nothing physically wrong with her. She was given steroid shots to possibly stop the psychological licking, but that did not seem to work.
Some cats lick their abdomen when they have cystitis or other causes of abdominal pain.
We have seen this resolve in cats with bladder stones once the stones were recognized and removed. Some cats do seem to do this as a neurotic type behavior. Using anti-anxiety medications or identifying the underlying cause of stress can help in resolving the situation in that case. Ask your vet about the possibility that this behavior may be related to cystitis or abdominal pain and about medications for anxiety if you think that may be helpful. I don't know of a topical medication likely to help.