If you leave someone for another person, don't be surprised if that person leaves you for someone else.
Some people just like to have their cake and they like to eat it too. Never make someone a priority that is only choosing to make you an option. You are better than that! You are a unique soul, a temple of righteousness, and a miracle of the almighty. Your life on Earth is limited and it is because of this that you shouldn't waste your time by giving it to others who have no desire to return it in an equal manner.
Though every situation is different, usually when someone is willing to leave you for someone else, they do so because they think of you as less than. They don't know your worth if they are willing to leave you to see if the grass is greener.
Love is complicated, and it will make you want to hold on to feelings that you maybe shouldn't. Take a different stance, and try to look at your situation from the outside looking in, maybe then you will be able to make the best decision for yourself.