AIO after finding out Husband has been texting “friend” over 20 years

I (73F) accidentally picked up my husband’s (65M) phone last night. In doing so I saw a text message from a girl we knew over 20 yrs ago when we lived elsewhere. He reacted badly & immediately wanted his phone back & did not want me to read the messages. Turns out he and this woman (54F) have been online friends since we moved. He contends he is not cheating on me, that it is just a friendship but I had no idea she was still in our lives. I am feeling completely betrayed. We have been married 38 yrs & I have kept zero secret “friendships”. He says I am over reacting & he is sorry, that he never thought of it as cheating & kept it a secret for 20 years. He still refuses to let me read the texts. I am seriously thinking of divorcing him over this.