Fixing this dress

I just bought this shirt at target and I’m really pleased with it except for how it fits around my armpits. I have smaller boobs in proportion to my weight so there’s too much space there and one of the straps keeps falling down. This rainbow shirt is the only white shirt I own. Should I buy a new one or is it good enough? I’ve never sewed anything and am scared to. Also sorry lol I haven’t shaved or cleaned my room in months

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