I'm impressed that you posted this question. That took guts. Lacking context for your question, I'll venture out on a limb:

If you don't advocate for yourself and your informed opinions then others will get the opportunities, raise, promotion and recognition you deserve. Sort of the snooze-you lose scenario.

The times I've pushed back when I thought I deserved a promotion, higher salary or bonus, I've never had a boss begrudge me for making the case for myself. Sometimes I was convincing, sometimes not. You have to be a professional about it. At the least, you'll put yourself on their radar for next time and hopefully get useful feedback on your performance.

As a boss, I've never begrudged anyone who made a reasoned case for themselves. Sometimes they prevailed, sometimes not. Some considered chutzpa is admirable as a leadership trait.

Asserting in meetings depends on the players -- whether the participants are all peers or if there is hierarchy present. In any event, listening (not just hearing, but really concentrating on listening) is a critical skill so you can guage when to assert an opinion or pose a strategic question.

I heartily second OldBaldGuy's recommendation on speech class. Public speaking is among the most terrifying prospects for most people. If you can gain confidence in addressing a large audience, small ones such as you'll find in meetings should become easier.

Good luck. If this is career-related, I'd Google a lot and see what guidance comes up.

Knowing that you need to be assertive is at least half the battle. Some folks never figure the first part out.