Sometimes it is okay for someone to walk out of your life. You may have reached a time where you are no longer supportive, instrumental or matching to their goals, standards and expectations. As in, you may have been a block to their next step in their life. It is only fair that they should leave and find someone else that would help them achieve what is next for them to.
In the period you have been together, you may have done what is required for that until you have reached your limit and you cannot do much anymore. This also goes to those who are rejected during dating and stuff. In that course you may have proved that you are not worthy of being an instrumental person in helping the other become a better person.
It is never easy and never ever will it ever become to part with someone especially that was close to you in all ways. The imagination of starting a new life all over again without them, facing new days and challenges and laughter and fights and dreams, hopes and expectations. Gosh! But never lose heart, you will get someone else in whose life you will have the capacity to influence them to reach greater heights or the same being done to you.