Your adult clients do take you seriously.
Your self-confidence is low and experienced (older) people will sometimes use that to gain a price-advantage over you.
Take a deep breath, and ask for more; but, be prepared to either lose the "contract" or negotiate the price. You'll get better with practice. Some choose to use an agent to do all that stuff for them or in addition to what they find on their own.
If you ask for a penny, no-one will pay you a pound.
Everyone here has had the same experience when we began our career. We have all felt undervalued at some point. (Maybe I'm not as good is I'd like to think I am. :/ )
That said, there are many different ways to be reimbursed for your work and talent.
Money, cash or negotiable instrument, is most convenient means; but, it is not always the best value. By that I mean that you can sometimes get some thing of greater value than if you used cash to get it. You sometimes can also get some service of greater value than if you used cash to get it, etc. If you cannot negotiate for full value, you have the ability to ask for significant discounts for consideration. Occasionally, negotiating involves "loss-leaders" or the commitment for a more lucrative opportunity at the "expense" of the current one.
Everything depends on what you and your client have in common.
Be flexible as well as being creative.
For two examples:
I negotiated a contract for free electric power from the power company in exchange for design. I got a credit balance with a monthly debit that draws down the balance. At my electricity rate, it's been pre-paid for years.
A colleague (and family) has lifetime free pizza in exchange for their box design, (a part of which they re-sold as stock-art at a profit. You've all seen the red line-drawing of the round-faced chef with the "ok" hand-sign)
There are more stories of payment in kind or other forms to use for examples.