First of all, it is necessary to deceive the partner objectively and impartially weigh all the "pros" and "cons", and decide whether to keep their relationship? If heartache is still too strong, and emotions interfere quietly ponder this question, it is better to postpone the decision for some time.
In the event that, despite the offense, bring to the gap is not desirable, it is necessary to deal with the greatest care: what prompted the change to the partner? Try to keep at the same time from a completely natural and understandable temptation to shift the blame to the other side! In reality, almost never so guilty of treason was only a partner, or of the partner. Honestly and objectively check their own mistakes, blunders. This will help you to improve relations in the future.
Be sure to try to speak frankly with his "half". Do not hide what you really offended, feel a strong emotional pain and, at the same time, recognize its share of the blame. Assure that will make all the necessary conclusions and try to avoid the same mistakes in the future. Speak only substantially straight and clear, in any case does not break down into accusatory, do not put a partner in the position of the accused, acquitted. Especially refrain from insulting, offensive words and all of the "go to person".
If you yourself feel and understand that without assistance you can not do, consult a qualified psychologist. It is not necessary to seek advice and help from relatives, friends, girlfriends: for good reason, they stand at your side, their opinion will be biased.
If the offense and heartache is so strong that the preservation of the relationship can not be out of the question, try to at least part decently, without scenes and scandals.