Anyone you work for is also a person. You can talk to them as such. You can be transparent with them, and say you felt that you erred and are undervalued, and you can tell them that you feel your lack of negotiating skills has put you in a poor position.

You'll often find that when you approach someone with honesty and transparency, they usually respond in kind. And their honest response may be, "Sorry, we made this agreement and you need to choose to live with it or not." But they could also surprise you and say "We agree. Here is a raise." But the most likely response will be somewhere in the middle, with some specific plan laid out to hit performance milestones, and in return get a raise.

Negotiation is not a one-time deal when you accept an offer. It is an ongoing conversation through your entire career. That is how long-term retention happens in an organization - with both sides communicating their needs and deciding on long-term goals and compensation.

Most of the other comments here take a more adversarial approach. And while that is effective, it sets you up for these long-term conversations to be forever adversarial, instead of collaborative. It is your call which kind of relationship you want with your employer.