Watch your endings, your politeness should match. 
I realize that the grammar I’m learning might be a little too hard to correct for beginners, so I’m going to be giving my sentences to my Japanese coworker to correct and post my humilia-ahem- corrections here for anyone else to learn from.
日本語の新しい文法を使った文は作り甲斐があるけど、そんな文は書きかねます。
It’s NEW grammar not NEW Japanese, you idiot!* Change grammar learned to grammar used. けど is pretty formal so you’re ending should match, use ます！ And here you’re telling newbies to keep the correct politeness throughout! Hypocrite!*
毎日自分に「今日こそ日本語の勉強をせねばならない」と言うというものの、ずっとゲームをしてしまう。
You’re using a really formal/literature grammar like ～ねばならない and you just decide to drop the を like you’re just having a casual conversation? No! No!* And now you have 2 を’s better change the first to の so it looks better. Also you’re obviously feeling regret all the time so change する into してしまう. You should know better by now, stop being lazy* with just using the dictionary forms of verbs!
*May not be actual comments from my coworker but conveys the feeling all the same.
)
On a positive note, I was able to read your sentence now and understood it too !!!
しかし、ワニカニと文プロで勉強していた。目標は毎日勉強する事。