Working together is a wonderful part of being in a relationship, but your partner tends to overestimate your abilities and your resources as a couple, and you probably get swept up in their grand vision. They talk you up, whip you into an excited frenzy -- and then you both find out, sometimes too late, that you can't really accomplish what you thought you could. You're competitive together, though, and you want to take on big projects -- even when it's not the best idea to do so.
This comes from a good place, in the sense that it results from a feeling of positivity and expansiveness that grows whenever you're around each other. Your lover, especially, thinks that you can tackle anything as a team; they just have to realize sometimes that that's not exactly true. Your beliefs and ambitions might not match up as well now as you thought they did in the beginning of your relationship, and after a while, it may have become apparent that you're working at cross-purposes. You two may be surprised from time to time when you think you've been working in tandem but find out that in fact, your efforts have been going directly against each other, in effect canceling each other out! Sexually, this could be a problem as well; you might be out of balance with each other in the bedroom, or your partner might go overboard in sexual expression, making you feel turned off as a result. As usual, compromise and making an effort to really listen to each other can help greatly.