Dear Thelma: Am I jealous or is he cheating on me?

I'm worried she will try something and he won't stop it or realise it; we have had arguments over her. She just makes me feel uneasy. I have asked why he is speaking to her, what the gain is. He said that he wanted a friend and it's nice to have someone that’s thinking about him and nice to see messages from her.

He tells me he always replies to me first and he enjoys talking to me more. However I can't help but have this fear that she is more or she is trying to be more and I feel like I am not being listened to or understood. It feels as though he isn’t willing to message her less although he tells me it's only a couple times a day. I can't help but wonder if it's more and if he is lying to me about it all.