Some sort of struggle between the two of you, subtle or overt, will pit your true self against your lover's path in life. Somehow, you both have needs or aims that are at odds with one another. This influence may not have been apparent at the beginning of your relationship; but its negative effects -- if they've even shown up yet -- were an unhappy surprise that popped up after your feelings for each other had already deepened.
You two probably started off feeling perfectly in tandem -- trusting of one another, sharing goals and ideals. Much later on, it became more apparent that you're following two very different paths, ones that may even work at cross-purposes to each other. You simply have different views of the world, or different ways of approaching obstacles and challenges. You could feel dragged down by what you perceive as your lover's pessimism or weakness, or you could feel as if their aims in life are in direct conflict with your own. What's more likely is that it's all a matter of perception -- that what looks like weakness is simply a different personality from your own; or what feels frustrating in your partner's life choices is actually those choices serving as a frame of reference for what you don't want to do in your own life. You could stand to learn from one another, but struggle and frustration are more likely. The two of you have to decide if you want to overcome this, or let it drag down the relationship.