Disappointment When Goal Fulfilment is Delayed I am wondering if there's a term in psychology for a situation where you want something really bad, but you are not able to get it for the moment (or a goal that you are not able to achieve) and when you eventually do achieve the goal or get that "thing", you feel unhappy, it's a really bad feeling because you're maybe too late on something.

Here are two case scenarios that might be unrealistic, but I'm sure you will get my point:

1st case scenario: your dream is to buy a convertible car to go to the beach with your college friends, you spend 10 years saving for that car, and when you buy it you realize you're not in college any more and all your college friends got married, now you're not happy and have this bad feeling in your stomach; you wish you had the car 10 years ago.

2nd case scenario: a new gadget is released, all your friends buy it and mess with it, your dad tells you that he will get it for you next year when you get good grades (or for any other reason). Next year comes and you got good grades, your dad gets you the gadget, but nobody is talking about it any more, you're still unhappy although you have the gadget.
I can think of a term, but it's not psychology-specific: unmet expectations (that and lack of attention).  Your two case studies are basically the same in these aspects.  

Unmet expectations are just that: we expect a certain emotional state when a situation is achieved (like buying a new car) - if that state isn't fully reached (which it rarely is - life is so rarely as clean cut as our minds would have it), our expectations are unmet and we are left 'to make up the difference', so to speak - disappointment, anger, frustration, all of these are possible, and more.  

How important we rate the outcome is also important - and unmet expectation isn't a big deal emotionally if the outcome isn't very important to us.  This is similar to reading a book and then going to the movie adaptation of it: if you have a mental picture of how the movie 'should' look, then you may be disappointed with how the actual movie turns out.

The other half, not enough attention, is also quite straightforward: if we are paying attention (the right attention, of course) we can see that material possessions rarely bring lasting happiness (this is actually a function of a remarkably adaptive human physiology, but I don't understand it well enough to explain it over the internet, sorry).  And with attention, we know that while we may enable more experiences, fun or lasting experiences, with material possessions, pursuing the thing itself as the end, rather than the means to the end, will inevitably result in dissatisfaction.

Lastly, there are a lot of popularized articles about expectations.  Here are a few:
Psychology Today article
NCBI Paper: The effect of unmet expectations among adults presenting with physical symptoms.
Tiny Buddha: Unmet expectations

Most of these were found by Google Search for phrase unmet expectations, so there's a lot more where those came from.