Heartbroken by Old Messages on Husband's Phone

Assalamu alaikum. I found some old messages on my husband's phone from when we were engaged. He was texting a sister he had met before, asking if she was free because he was in her area. She worked at a cafe, and he mentioned it was near his place. They exchanged a few messages back then. I know it was wrong to look through his phone, and I feel guilty for that. He is known to be very practicing-he prays all his salah diligently-so this has really shaken my iman and hurt me deeply. I don't know if they ever met, and we've only been married a few months; our marriage has been beautiful otherwise. This has changed how I see him. Please, what does Islam advise in this situation? May Allah guide us.