If you're in an unhealthy relationship, you may be held back in other areas of your life. For example, let's say you're in a relationship in which you are unnecessarily clingy and needy. You're looking toward your partner to "make you happy" by giving you Get Her Hooked continual attention. Your primary focus in life is on your partner and your life together. This codependence can hold you back in life and cause a host of relationship problems. It's not your partner's responsibility to make you happy; it's yours. Those that struggle with codependency tendencies need to begin a journey within to seek fulfillment and stop looking toward others to fill that void.
Another example of how a relationship can hold you back is if your partner has settled in certain areas of life and expects you to do the same. Do you have more you want to accomplish in life, but your partner thinks you're crazy for having such goals? Do you feel pressured to settle or just live according to status quo because your partner is? If so, your relationship is holding you back and it's time to sit down with your partner and have a serious discussion. Partners don't have to have the same goals, but if one wants to grow and accomplish the other ought to be fully supportive.
Remember the days when you were young and your parents told you to choose your friends wisely because you become who you hang out with? This philosophy is true in adult relationships as well. If you choose friends and/or an intimate relationship with those that are positive and interested in professional and personal growth, you'll be more apt to be interested in those. On the other hand, if you spend time with those that are unmotivated or negative, it could very well stifle your growth. Keep in mind that when it comes to your relationship, it is desirable that both partners have the desire to grow professionally and personally, as this balances the relationship.