<p>Is it gramatically wrong to say that something "has meant a great deal FOR me"?</p>
<p>It may be technically grammatical, but it's awkward. Use "meant a great deal to me" or re-word the whole thought.</p>
<p>"to me" and "for me" have somewhat different implications. Think carefully about which meaning you really are aiming for.</p>
<p>It may have been important FOR you, or it may have meant a good deal TO you; it may have provided a learning experience FOR you or been an opportunity FOR you to grow; it may continue to be WITH you (like the force), it may have been given TO you; it could have been done BY you, it could have been done FOR you, it could have been done TO you...</p>
<p>to would be my choice given the context you've provided</p>