Well for a start, yes you are being a dick to these girls. Wether you can help it or not, it's not exactly fair on them. Then again it's not fair on you to stay in a relationship your're not happy in.
How often has this happened? You say 'every girl' but how many girls have you been with, like one or two? Or numerous?
It could be that you're not actually all that much into the girl as you think you are at the beginning, and just the thought of starting to date them excites you, and once you get them, you realize that you're not into them as much as you thought, not that you get bored easily.
And how soon do you get bored with the girls? Like a few days after, weeks, months, etc?

Well.. when you are not serious enough, and just for the sake of fun, you are doing this, its really vulnerable!! You should stop this or else, in such kinda fun, you may loose your loved one and may not recollect the person again who is suppose to be the best in your life!!

^oh cut him some slack.. he's just 17..

I suppose it happens because you weren't really in love with any of the girls.. I don't think there is anything to be fixed.. you're young it's normal to want to experiment and I guess you enjoy the challenge of "getting the girl" .. and probably you're just not ready for commitment yet...
Just enjoy your life and try not to hurt the girls too much.. I think it's better to end it sooner then lead them on and make them fall in love with you if you don't share their feelings...
Anyways... you shouldn't worry too much about not being able to stay with a girl for a long time.. when you will be in love you will not get bored and you will think the girl is perfect you have plenty of time for that... I think that everything comes at the right moment

I'm exactly the same man, i'll end up with the girl hanging on to my every word, and I start off really excited and interested too.. then just as I can see the end of the 'chase' and a relationship starting, everything in my head goes boom, and suddenly i see a million flaws, and a million reasons why we shouldn't be together.

i used to be the exact same, like it was the thrill of the chase more about the girl, but i knew this and decided to not get into any relationship because id probably end up hurting the girls i was with, if your only 17 then its perfectly natural to me, your young, you want to go out and have fun, for me i changed when i was about 18 and i wanted to start getting into relationships, its all down to the girl really, how much you like her, it might be that you are still young or it might be because you dont truely like the girl to be in a relationship with her. how quickly do you get bored in the relationship?? like a few days, weeks months?? if i were you, i would try and not get into relationships unless you are sure that this is what you want. i would say just try and find the right girl for you.

I agree - you're young and it happens to a lot of people. You're not being a dick; I think it's worse to let the relationships drag on. It'll probably change when you get older, but if you want to do something about it now, you could always try to get to know the girl a little more before you make your relationship official, and think about what it would be like to be in a relationship with her. See the flaws ahead of time.

The boredom could be because you may be "settling" after the chase is over. If you think trying to get the girl is exciting, find exciting ways to try and keep her. It may not be that she is boring, just that the relationship is a little dull. She has to do some of the work too of course - just make sure you don't rely on her to keep it entertaining.