If your boyfriend is dating someone else, then he's not your boyfriend anymore!!!
You may love him, but he obviously doesn't love you no matter how many times he says it. Look at his actions! He's only going to do to you what you allow, so stop allowing him to have his cake and eat it too. Give him an ultimatum either it's you or her, then be out!!!
Take a nice hot bubble bath, cry your heart out, listen to some motivational music and bring back that confidence and self-esteem you probably think he stole from you. You have the upper hand; stand firm and make the best of it. Good luck!!!