Partner's Porn Use Shatters My Trust

Last night, I was playing a phone game in bed and I got up to look for my charger. My partner was getting dressed beside me and their phone was on the bed screen up. I saw a naked photo of a woman, open on their phone. They saw me staring at it, and they acted just as shooked as me. They said that someone had retweeted porn and they must've accidentally clicked on it when they got up to get dressed. They had just taken a shower when this happened. I don't care if they look at porn or if they masterbate to porn. My issue is that I have already been really worried about my partner cheating on me. I've talked to them about this several times before. They always say that Im the only one for them but im starting to lose trust in this more and more. Before this, I was finding things in our house that made me suspicious. Things like carmax, clothing pins, and a weird drawing. All these items they claim to know nothing about and pass it off as something else. We have been together for 3 and 1/2 years. I don't know what to do. I love them, but I can't help but feel like they are lying.