Am I over reacting to old messages?

So when my fiance, and I started to see each other we both were actively talking to other people since we just met. According to him when we first met he was speaking to another woman, and he eventually ended that whenever we decided to be official. I knew all about her he was straight up, and honest as was i about my other fwb.

I thought his lady friend wasn't a serious fling. I am a stay at home mom, ( yes stepmom even though we are technically married), I was letting the boys play with my phone. I decided to find his old phone to give to one of out boys, my mind playing devils advocate of course led me to his text messages. I go to through the messages to when we first met , and went to their old text thread. My heart broke, because what I assumed was a fling he dropped for me he was telling her he loved her the same time we were first seeing each other. Should i be upset about this or not..maybe i am just being emotional? We are ttc, and i love him with everything am I wrong for letting the past control my present?