Rachna has low self-esteem since the beginning of the show Vihaan through his fake attention made her feel beautiful and confident. Even then, he still compared her to Gunjan alot.
That would obviously hurt her ego. Then to top it all off, he chooses the engagement day, in front of family, friends and neighbours, to say that he does not love her and prefers and loves her cousin.
I think that even a person with very high self-confidence would be shattered for a while and question themselves about their "short-comings" in such a situation.
It would take some time, but eventually, Rachna will realise her worth.