I have to agree with River on this one.
You see, when I first started exploring this aspect of my experience as an early teen, I unwittingly conducted an interesting experiment. As an impatient youth, I was quite frustrated by the vague and cryptic terms of my precognitive dreams (which to assuage your pessimism, I am actually aware that there is something different about my precognitive dreams *WHILE* experiencing them... and as such, I make a point to remember them and attempt to decipher them for some time once I wake up). So I both consciously and unconsciously expressed a desire (in many different forms) for my dreams to be more precise and accurate. Much to my surprise and initial glee, my wish was granted.
So what does this have to do with causality and inescapable fate? Well, it wasn't long before my glee turned to dismay, as you see my dreams were not only more symbolically clear (to the point of being downright literal), but they were also more *immediate*! I would wake up sometimes to the beginning of a chain of events that I had just dreamt. Given that my adolescence was a largely unhappy and miserable time for me, it was quite frustrating to me to be given a preview of just how miserable my day was going to be. So I began to attempt to change things. While my initial attempts failed and led me to conclude something along the lines of the "uncurbable river of destiny", my experiences as I've grown older have changed that idea quite a bit.
You see, I dream about the important elements of my life well in advance. The people, places, purposes. I remember them (the old fashioned way, I'm a bit wary of writing things down, though the enthusiasm for it here is making me reconsider it) and have gotten much much better about not only understanding them, but also being able to put them "to the music" (so to speak) much sooner than I used to be able to. Like our sceptic from above pointed out, it's a natural function of the human brain to realise we've dreamt about something immediately before it happens. What he failed to mention however, is that there's nothing silly or mistaken about it when it is a dream that you had spent a lot of time thinking about or attempting to recall *BEFORE* the event happened. But that's neither here nor there.
In all of my learning how to interpret and understand my dreams better though, I've discovered that (at least in my own case) I *CAN* screw things up by not acting on them properly. (I say screw things up because I'm most concerned with bringing to fruition the rewarding and fulfilling glimpses that I've had.) But that cuts both ways as I also *CAN* avert things I see happening, and actually have learned to do so reflexively and intuitively as I'm dreaming, so as to test the viability of my reactions! It's an interesting thing, and it has seemed to provide me with good information on how I can change events before they turn out to my disliking.
The one thing I would suggest though (which I think is one of the things that I failed to notice when I was younger), is that simply because you fail to change an event you foresaw in a Dream, it does not have to mean that that event was unalterable. There is the decided problem with only realizing that you've dreamt about an event *as* it's happening, that you may be left being able to do to little, to late.
To pose a hypothetical situation; say you've had a dream about walking with a loved one down the street at night and being accosted by an armed robber who hurt your loved one. If you don't recall the dream until the robber has already approached you, your options are quite limited. If you realise your situation upon first seeing the robber, you've got a little more room with which to work. And if you realise your situation as you turn onto that street, or even go out for a walk at all, you're options are much more open, and provide you with a very favorable chance for averting that stream of events.
The only question then is whether you've convinced yourself that the robber would have sought you out, or whether he would have kept on his merry way. It's good to keep in mind when asking that question though, that not *everyone* is sensitive to the possibility that lies before them, as most folks simply deal with the concrete aspects of reality that they can see, hear, and touch. I've experienced a definite power (and risk) in freedom of choice, and I hope that it can benefit you too in providing you with an idea of the experiences of no longer living in fear of the future you may have seen.
I hope that all goes well for you.