As a recovering Nice Guy myself, and one who helps a lot of Nice Guys recover from this, I can tell you my experience.

Nice Guys often suffer from enmeshment trauma, which leads to detachment from their true self in childhood. They often look to women to provide their sense of self-worth, like you said, because they were taught that their value is determined by their ability to please someone else (mainly Mom).

I see it over and over--myself included. Enmeshment with Mom leads to a value system determined by a woman. The path of Nice Guy recovery starts with re-establishing a connection to who they are, what they want, what their needs are, and establishing clear boundaries for themselves--internally and externally.

This includes filling their bucket from a variety of sources instead of hooking up an emotional hose to a woman and expecting her to fill that void. That void is almost always a psychic attachment to Mom that often goes unrecognized unless faced head-on.

Guys, if you're reading this, look at your relationship with Mom. Did she rely on you for emotional support? Was your father absent, emotionally or physically? Do you feel like you have no value if a woman is not interested in you? If yes, you need to re-connect with your masculine energy and realize that it's not a woman's responsibility to take the place of Mom or shield you from her.

Nor should she be the recipient of your hidden anger towards Mom, cause Nice Guys have a ton of it.