You have two options: believe him and get over it and move on; or don't believe him and end the relationship. I think you need to trust your instincts on these kinds of things. Maybe it really wasn't meant to be taken seriously, but that is not an innocent text message. There's no way to know for sure what his intentions were by sending it, but clearly he's acting inappropriately with other women, and not hiding it well enough if you found it on his phone. In my experience, your gut instincts will always lead you in the right direction. Pay attention to what your feelings are telling you and do not try to talk yourself out of how you really feel, just to stay comfortable where you're at. It won't do you any favors to sweep this under the rug because you will either build resentment over time, or he will do it again; and both scenarios will ruin your relationship.
You can either choose to believe him and get over it, pick yourself up and move forward; or you can break up with him and move on... either way, you need to figure out what is best for yourself and then take the steps necessary to make that happen.
Sorry to hear you're sad. Best of luck to you!
"Caring is not an advantage."