Trust..

In a relationship, you learn to trust your partner. It takes awhile for your trust to build up in one another but that's only because you are beginning to learn about a that person. What happens if you've been with a person for almost a year now and some random girl messages me and says he has cheated on me with her? He admits he hung out with her one but it was nothing more. Her story said their was alot more. He still denies it. I understand that I barely know this girl but why do I keep leaning onto her story? How do I know he is telling me the truth? It's hard. I want to believe him but then I'm looking out for my heart.

HELP! I need some input on this, please.

xoxo