The thing is you can be alone and be by yourself and you really do not need to be in a relationship to be happy. Because if you think about it, you are in one now but still not making you happy.
The only reason you are keeping a relationship with this guy is because you want his confirmation. The fact that he is dating other women, he is making you feel like there is something wrong with you, and all you want to do is get him to notice you and want you only and no one else.
He is dating other women because he wants his ego stroke. It doesn't mean that you aren't good enough, it just that he is insecure with himself.
My advice is to look deep within yourself, ask what truly makes you happy and who can you be without him in your life. Go take care of you, pamper yourself, and only do things that makes you feel happy and comforted.
If you continue to see him, accept him for what he is, and do not try to change him, because you won't be able to. Instead, change your thinking, and focus on you instead of him. Pretty soon, you won't even worry about him anymore. And you won't worry about any other guys for that matter. The important thing is you, not him. Take care of yourself:)