That seems serious to me. Do you know how much it takes for him not to lie to you. If you would leave because he said this than you aren't head over heels for him.
Not sure if I believe it's 100% true, but I read that men and women fall in love in two different ways. A woman's love for a man can grow, she can end up in love with someone she never expected to. A man feels a physical spark immediately, if he doesn't get the spark, he'll probably never get the spark.
You are the only one that can decide that. Do you think he loves you at all?
He may never get over the love he can't have and that will always be an issue. Is having only part of his heart what you want, because a part was given to this over woman and it doesn't seem he is ever going to give it to you fully.
is he loyal?
did you tell him it bother you?.. is he caring? you need to ask yourself these question then decide on what you want
when you start questioning your relationship then there is something wrong in it and you should question, should you stay or should you go?
do whats best for you and dont be in a stressful relationship,, the reason we have a relationship is to make our life more easier and enjoy it together. . if nothing had changed since he promised you then nothing will ever will