What so many people have clearly missed in the episode was that the mountain knew about the plan with the grounders inside and they took them and the remaining 44 prisoners. Lexa had 2 choices: fight and her people and the 44 would've been killed or guarantee her people's safety at the risk of killing Clarke's. She chose the option with the best outcome and like she said it's not what she wanted and she's sorry it had to happen that way but it she didn't have another choice. Now maybe she goes back to help or maybe not but she did the right thing. Lexa's decision had nothing to do with Clarke "rejecting" her. And on that subject Clarke never actually rejected her, Lexa asked her what she planned to do after their people were safe and Clarke said "I don't know I can't think past today" and THEN Lexa asks her to come to the Capitol with her saying "it'll change the way you think about us" to which Clarke replies with a smile on her face "You already have" and Lexa smiles a little at her response. Later when Lexa is telling Clarke about the deal you can see the hurt on Lexa's face cause it's not how she wanted things to go down and she loves Clarke and knows how much it's going to hurt her and that's not something she wants but like I said she only one choice that guaranteed the safety of her people so like she said she had to make the decision with her head not her heart and before she leaves she says to Clarke "I hope we meet again" which further proves what I've said and may also mean she'll come back to help maybe not either way Lexa's decision wasn't about revenge on Clarke for "rejecting" her it was simply the only logical option presented to her and I believe that in the back of her mind Clarke knows that and agrees with Lexa's choice. Trying watching the episode again now that you know what's going to happen and won't get caught up in your emotions on what happens and you'll see what I'm talking about.