it happens to everyone.

Actually,the answer is really simple.

Its the game's problem,not yours.Not being bothered to finish a game means that it doesn't interest you enough.At first its fun but slowly it loses its magnificence.SPM is a great example of a game that starts out greatly but gets bad in latter levels.The answer to your problem is buy different games.Stop buying Zeldas if you cant finish them for if you can't finish 1 you prolly won't finish the others either.Make exceptions and try 2D Zeldas or MM which is considered greatly different.Be more selective when buying FFs as they all are very different.Personally,although I've played most of them,FFX was the only one to have me fully hooked till the very end.
Generally I think you should alter your game purchases criteria.

All of the above stands false if you stop playing a game due to being stuck.

Ok, maybe that´s it! Maybe I should try something new and different for a while!
But then the question is what?

Try Suikoden!

Some problem sometimes. I can chug a game for 14 hours non-stop, and then not play it for days. I usually return to it in two weeks.

What kind of game is it?

And how much is the price if you know?

Paper Mario owns ^^

It's a turn-based JRPG series, but it's different in the way that you never have to save the world, instead they focus on a conflict between two enemy countries, or a rebellion in a country, so it's much more believable. Also, in every game your objective is to recruit 108 characters (most of which are usable in battle)

As of price, I have no idea, I know they're pretty rare, but as I live in a country where it's impossible to find an original game (let alone that when you do, it costs twice or more what it cost in the US), so I an't help you there

I live in norway so the prices here are really really bad! It´s so expensive to buy new games in the stores here!

I´ll check it on www.play.com and www.ebay.co.uk to see if I can find an ok price.
Do I have to play number 1 first, or is it the same wich one I play first?

Depends where you start. The last one, V, stands completely on it's own (in fact, it's the perfect starting point), II and III depend a bit from their predecessors, and IV shouldn't be played till you're sure you love the series as it may lead you to think the games are mediocre