I'm new to these forums.

My issue is not really with my fiance but it affects us, and I want to be able to fix it.
Here is a little background. I have a sister 10 years older than me. and about 6 years ago her husband left her and she was heartbroken, and depressed, even suicidal. I moved back in with her and her daughter and did everything I could to help, I actually forgot about myself. I had gone through some therapy at the time, and realized I wouldn't allow myself to be happy if people around me were unhappy. And I did my best to deal with that.
Fast track 4 years ago my sister remarried and I moved out. I also made a decision to leave my religion and it's strict lifestyle, which my sister and parents are still apart. Following the religion for them means that should cut off ties with me. It hurt and I felt guilty, but I was finally ready to live my own life. I met and fell in love with a wonderful man from the USA (I'm in Canada) and we are engaged, I will be moving there within the next few months.

My sister and parents started talking to me again and we have somewhat come back to normal. My sister is going through a separation with her current husband and once again is dealing with drama. She didn't even come to try on wedding dresses with my mother and niece. And it hurt my feelings. My problem is I can't let go of the guilt feeling of me being happy, and I've been lashing out at my fiance for no reason, I am more aggressive and very hard on myself. With the help of a close friend I was able to asses that she was my problem. Well not her because she won't change, but my reaction to her. It's not fair to myself or my fiance. Why can't I allow myself to be happy?

Just needed to vent and am hoping for some advice.
Thank you