Not exactly where to put 以上 in the translation, here's my take
雪菜のためにがんばらなくちゃと思うと、自分の実力以 上の力が湧いてくるんだ。誰かのためにがんばれるって さ・・・自己満足かもしれないけれど・・・こういうの って、うれしいことだと思わない?
I thought I would have to try harder for Yukina's sake, and because of that I was able to muster up my true abilities. For who's sake was it, that I was trying so hard, I wonder... It might have been for my own satisfaction.. but something like this, it's something that's supposed to be thought of as happy isn't it?
そうね。こんな私でも・・・慎一さんの力になれるなら うれしいわ ( idk what that name reads)
Yeah that's right isnt it, I also am happy if i become 慎一さん's strength/power.
雪菜がいるだけで、僕はすごく幸せになれるんだぜ
Just by yukina being here, I'll become very happy.