Why can't I move on from the death of my mother and why I feel so lonely after she is gone?

My mom died due to cardiac arrest almost 3 years ago. At that time i was 19. This incident shook me completely and changed me. I felt that my friends and relatives moved away from me as i became a very insecure, scared and pessimistic guy.
One month later to this incident a girl from my high school contacted me. And almost after 1 month she confessed that she likes me. I told her about my situation and i told her that I am afraid of letting her in, giving her a very significant position in my life and when she'd leave I'll be hurt again. To this she said it's fine, i can take as much time as i need. And she said that she would even help me move on. After few months i started sharing my feelings with her. Whenever i would miss my mom I'd tell her.
Things would sometimes feel getting better. Sometimes I'd rewind everything and go back to the day i was in when my mom died.
One day when i told my girlfriend that i miss my mom so much and I can't move on. She said that bad things happen to everyone but everyone doesn't keep crying and whining about it.
Almost 2 years passed and she left me. And got married to someone else. Although I always asked her if she is sure that she was gonna marry me and she'd promise me that she would marry me. so this way i lost someone with whom i had shared almost everything that i was and i am going through. And now i am left alone. And i fear that no one will accept me and stay with me forever. I crave affection. I crave someone who would tell me that everything is gonna be ok. I crave someone who would show genuine care and concerns towards me.
Why can't I move on from the death of my mother and why I feel so lonely after she is gone?
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