Good that you are using the Estriol cream nightly at present.
The Interstitial Cystitis Association website has a page on Antihistamines you may like to look at. I can't paste in a link as there is commercial content on the page, but this is an extract:-
"Though the most widely used antihistamine to treat IC/BPS is hydroxyzine, some people with IC/BPS find relieve from Claritin, Benadryl, and Singulair."
My Gynae didn't specify antihistamine type. I tried Fexofenadine last year for other reasons (sleep) but it was post-gynae surgery when my GU tract was completely messed up following numerous courses of heavy duty antibiotics & consequently antifungals + 8 weeks without any vaginal HRT - a nightmare scenario for GSM! I don't recall any improvement in the GSM while taking Fexofenadine, though was not on it for long at all.
I've got a tight/weak pelvic floor + also a mild bladder prolapse (along with the va +IC)!!!
Ouch, that's a shedload of GU/PF issues for you to manage, but sadly not uncommon at this stage of our lives. The HRT, vaginal & systemic, should help over time, taken at the right doses but if anything like me, you may also need to be vigilant with diet to keep your bladder comfy. I don't need to follow a strict IC diet most of the time, but I do avoid those triggers I know from long experience spell trouble for me & then tighten up some more if a flare strikes.
Joaniepat is very knowledgeable on bladder health & though I don't do Facebook personally, I think her suggestion is well worth following as you're suffering so much.
Wx