My S.O. felt the same way a couple years back, and despite my cajoling, she maintained her reluctance to ask for more money. When she was finally ready, she told her boss they needed to have a meeting to discuss things, and he said okay, but kept putting the meeting off. For like two months. When they finally met, and she made the pitch, his response was, "Oh, is that all you want, more money? I thought you were going to tell me you were leaving the company, that's why I kept putting this meeting off." She got the raise she wanted, retroactive back to the first of the year, and he made her eligible for certain bonuses she wasn't considered for previously. Now she's getting ready to have that conversation again, because she's working her ass off and believe she's worth more now.
The squeaky wheel gets the grease. Get over that reluctance and lay your cards on the table (and be ready to bolt if they don't meet your demands).