jealousy????

When my partner came into the picture I never realized that l could be so happy. We've been together for four years and she has never given me a reason to doubt her faithfulness to me and to the relationship. I am 36 and my partner is 30. She recently went back to school and while there she made a new friend. This woman is 49 yrs old, married (unhappily), and has 3 grown children. She and my partner have developed a close friendship and I'll be honest....I can't stand it. I'm so eaten up with jealousy and this is so unlike me.I feel like a high school kid who's best friend has made a new friend and is jealous. It's crazy! My partner has ured me that there is nothing more than friendship there, that I am still her best friend, and she loves only me. I know there is nothing going on but I can NOT seem to get a grip on this jealousy. Has anyone ever been in this situation and how did you handle it? For 4 yrs it has been just me and her. And now the new girl! ***PLEASE NO UGLY COMMENTS