What does this mean? My guess is "In order to prevent myself from getting too excited"?
If you can please break down the grammar for me to understand
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I'd say you are pretty close, but I would translate it as:
It seems I am the only one who did not get completely excited
Let's talk about the grammar and why I think this. Full sentence again for reference:
The grammar break down:
俺 = I/Me
は = Particle declaring that the previous thing (I/Me) is the overall topic
自分 = Oneself
だけ = Only
が = Particle setting the previous thing (only oneself) the subject of the sentence
勝手に = One's own way / Willfully
高ぶって = 高ぶる in the て form. Means to be / get excited and additionally XYZ. The additionally XYZ part comes from it being in the て form
しまわない = しまう in the negative form. しまう Means to do completely / accidently.
Taking it means "completely" here. So we are saying has not/did not do completely
よう = Seems
I say "seems" here because I'm assuming this is a complete sentence. Any left off parts can easily change the meaning
Now let's put it all together and convert it into "English":
I, only myself, in my own way did not excited and to completeness it seems.
"Cleaning" it up to sound natural:
It seems I am the only one who didn't get completely excited.
Here is another sentence break down which I got from 1 guy in Discord ,in case anyone on internet want a reference . This is just MC thought about his action, could be "excited about emotion /he's acting too fast because excitement
自分だけが Only myself
高ぶってしまわない > 高ぶる to be excited + しまう involuntary action + ない negative
よう in order to / so
"So I won't get all excited by myself"