So sorry you have depression, hope you are having support to recover.
This is proving very difficult to word carefully , as your partner has strayed in the past you are bound to be concerned
Are you more depressed because of his friendship with the woman or is he enjoying the company of the woman and the rambling club because he needs a break from your depression , carers can need a break no matter the illness , depression, disability , one of my sisters has Parkinson's , her husband loves her very much but he does need to do his own thing for a break.
I can understand why you think there is something going on, he does have history, you say a long time ago, how long?
Elegran would never hurt anyone, no one here would hurt anyone who asked for help. Do reply