I am unable to control my negative thoughts, please help?

for about 6 years now I have been experiencing mood swings intermittently. At times I am everything I want to be , loving, confident and self ured. but at other times I get very depressive and feel extremely alone and victimised. this is affecting both my career and relationship with my would-be husband. he is no longer patient and undertanding like he used to be and I fear he doesn't trust me anymore. he avoids me when I need to talk to a friend if I'm feeling low. I Love him a lot and I wish to keep him happy somehow, but I seem to failing at many fronts, what can I do to stabilize myself?