I am so glad Kara told Lena she is Supergirl. Kara wasn't forced to tell her, she wanted to tell her and told her on her own. Something to remember... Kara wanted to tell Lena last season but Alex stopped her. There were moments Kara has wanted to tell Lena, it is not like this was sudden either. At any rate, the confession felt real and powerful. Melissa Benoist knocked it out of the park and Katie McGrath was great as well. From a psychological perspective, given Lena's history (let alone, genes), I understand why she cannot easily get over this reality. Again, we are talking family genes and the hurt in general in different relationships she has endured. Kara is her very best friend. It stings. Yet, the second I heard Kara's confession (which made 100% sense) most (even a very hurt one with painful scars of the past) would forgive. Lena didn't. It's sad and frustrating. Not only will she not forgive or just need time to get over it... but as we see... it sounds like she's going to adjust her grand plan to ruin Supergirl/Kara.
We are talking major darkness collecting inside of Lena which sucks. Of course, I suspect she won't completely go dark and things will work out eventually but it's going to take a while for this storyline to play out. I just hope too much damage won't be done until then.