I congratulate you for having the courage to not only accept your problems but also to look forward to the needful solutions. Loneliness does create problems for many. People at times start developing certain insecurities, doubts or obsessions during stress. Remember, marriage is about mutual trust, commitment, respect, care, responsibility and companionship. It calls for mutual adjustments. But one should also try to respect certain boundaries and give some space to one's partner. Unnecessary doubts and enmeshments can harm relationships. I suggest the following:Do engage in certain constructive hobbies that match your aptitude / interests
Try to develop a healthy social circle if possible. Try to maintain healthy communication with family and friends.
Try to read constructive and positive books / literature
Try to listen to your fiancée and be encouraging in your conversation with her
Do not ring her at odd hours
Try to think positive. Try to challenge irrational ideas / thoughts with more rational and optimistic thoughts.
Try to maintain a healthy life style. Avoid alcohol / drugs of abuse. Eat healthy and sleep well.
Try to exercise regularly.
Try and maintain a constructive activity schedule.
Try to keep yourself busy in constructive / positive hobbies / work.If still finding it difficult to overcome your insecurities, I suggest you should consult a psychiatrist.