The first thing that you must do is to talk to your friend regarding the changes you have noticed. Be genuine, be empathic, and tell her that you will stand by her side and encourage her to share her difficulties with you. I gather from your description that the two of you have been together for three years and you clearly mention that besides the current difficulties she is liked by you, and you both plan to get married by this year. In such a case you should act as her support and help her walk through the difficult phase On the basis of difficulties told by her you may decide to seek a psychiatrist or psychologist. However it would be useful for you both to seek an appointment with a counsellor for adapting your relationship to the next level. Hope this helps you..