Feeling jealous if husband takes a second wife

Q: Allah created both men and women differently such that a woman does not naturally feel to have more than one spouse at a time.

1. My question is that if it was in the nature of a male to have more than one wife as created by Allah then why are there naturally the feelings of jealousy or insecurities and intolerance that a woman feels about it? My mind and heart both know that Allahs plans are the best and I blindly trust in them but then why I feel annoyed and really bad about the fact of sharing my husband with someone else. I even feel annoyed when my husband teases me about angels of Jannat? Why has Allah put this feeling in us knowing the natural tendency of males? 

2. Though I know that there is nothing wrong in having more than one wife but I still feel uncomfortable and jealous about this fact. I would never be able to stand the fact of my husband marrying someone else. Does that mean that I am not a good believer as my husband constantly keeps saying that if I cannot tolerate the fact of my husband having another wife (suppose) then I am not a good muslimah. Is that so?

Authenticity of the dua for Rajab and Shabaan

Q: I would like to know whether reciting the following famous Dua is Sunnah/Permissible/Bidah:

اللَّهُمَّ بَارِكْ لَنَا فِي رَجَبٍ وَشَعْبَانَ، وَبَلِّغْنَا شَهْرَ رَمَضَانَ

Is there a strong command or a sign of permission from Rasulullah Sallallahu Alaihi Wa Sallam regarding the recitation of this Dua in the months of Rajab and Shabaan?

Is the validity of nikaah subject to repentance of past sins?

Q: During childhood a woman committed sins. The sin was that the woman, who, in her childhood was a girl, rubbed her private parts on another girls private parts. The woman doesnt think she had underwear on when this happened but the other girl had underwear on. This sin happened either when she was 12 or under the age of 12. It happened before the age of puberty as she got her periods when she was 14. As she matured she realised that was a homosexual act and it was wrong.

The woman is married to a a good man, who has never done such sins in his past. and the woman has recently been getting doubts upon repentence. She read somewhere that if one doesn't repent for such sins or sexual sins before marriage then marriage to a person who has not done such sins in the past or marriage to a chaste person is haraam and not valid.

Was the marriage valid from the start? Was it haram to marry such a person who has committed such sins in the past when the other hasn't?