Need advice pleaseeee help ! 😭

Me and my fiancé have been together for 6 years now and just had a baby 3 months ago.lately I haven’t been feeling like myself which has caused me to be overly jealous and on top of him. He always says I over exaggerate but I recently found him liking girls pictures on Instagram and I realized that he messaged this one girl but continues to delete it. This isn’t the first time that I’ve caught him flirting with girls on social media/texts with him telling them inappropriate things. But every time I confront him he denies it to the max until I tell him what I found and he stays quiet. I’m so tired of his constant lying and I honestly don’t know what to do anymore especially because we have a son together. What should I do ? :(