I believe my partner is sexting other people

I am 58 and my partner is 70. We have been together for almost 12 years. We love each other deeply and are each other’s best friends and partners in business and life. For about 18 months we have not had sex. We have during this time, been through tough times financially and business wise, and with emotionally draining family dramas and deaths and the lengthy and complicated administrative legal issues surrounding these matters. In our discussions about our seeming lack of interest in sex which was concerning to both of us, as we had a very active and fufilling sex life prior to this, we put it down to the associated stress and feelings of lack of achievment and mental stimulation in our business environment. I was okay with that as I believed from what he said that he currently was feeling totally asexual. I respected that and as I have just gone through menopause and also understood exactly how he was feeling with regard to lack of achievement as I felt the same way. So it came as a huge shock to me a few days ago when we were engaged in whatsapp chatting as we were apart for a couple of days and he mistakenly sent an erotic image of a threesome from a porn site, to a Whatsapp group (business related) to which we both belong. Fortunately it was very early in the morning and as we were already chatting to each other on Whatsapp I saw it and immediately alerted him that he had posted inappropriately and better delete the pic immediately before the other members of the group saw it. Which he did. And immediately said that it was meant for me. I so want to believe that, as in the past whenever we were apart we would send to each other, images of, and links to, erotica, that we thought the other would appreciate. However, I just can’t imagine him sending me this, suddenly after 18 months of no sexual anything, totally out of the blue, whilst we were discussing legal matters surrounding his brother’s deceased estate. Therefore I immediately assumed that it was not intended for me, and that he must be engaging with a personal one on one intimate whatsapp chat (as we have done for years, and at which I know from experience he is so good) with someone other than me. This to me is totally different from watching porn, which we both enjoyed in the past. I feel devastated and betrayed. Sick to my stomach in fact and my whole self esteem which has been fragile given the changes my body has been going through as I age, is in tatters. He insists that it was meant for me, and there is no way I can prove otherwise. I told him I was ok if he was sexting hookers or porn sites, buy my gut tells me it is someone he knows personally. He denies it and sticks with his story that it was for me. How do I move on from this? I so very much want to, but I am shattered.