People always get bored after a while. Some people are just more committed than others, and sometimes the ones who aren't as committed are completely surprised by their lack of willpower.
A relationship isn't so much a being perfect for eachother than it is working to stay with each other. If both of you aren't working to keep the relationship together, then it isn't going to work out. Some relationships take a lot of work, some don't. What it comes down to is if you both feel as if it is worth the effort.
From what I have seen in your thread, it looks like you are possibly younger, which makes this next part very relevant.
I always had this idea in my head that I would find the right girl and everything would just be easy. But incorporating your life with another person is a big freaking deal and once it goes past dates and sex, it gets a lot more complicated. At that point the big questions start getting asked...
"Do I want to stay with him/her forever?"
"Would I be happier dating around?"
"I am not as interested as I was, just...comfortable. Is that what love is supposed to feel like?"
"There isn't as much passion anymore, maybe we aren't right for eachother afterall"
The thing is that people usually don't literally ask themselves those questions, but somewhere in the back of their mind, those concerns are getting bigger and bigger because they aren't adequately addressed. Pretty soon you find your GF or whoever spending less time with you, spending more time with other friends, picking fights for no reason, less sex, etc.
Those are signs that something is wrong. Girls and guys both subconsciously start acting that way because deep down they are confused as to if they really want to be in that relationship, and it is a shady way for the subconscious to deal with the problem. Make the conscious mind think that it is because of the other person that they are unhappy when it is really themselves conflicted. And the other problem is that knowing that something is wrong isn't the same thing as being able to get the other person to talk about the problems. That has to happen or else it will get worse. If the other person refuses to admit there is anything wrong, then it might be a lost cause.
What your GF says to you and how she really feels are probably completely different things. I bet you her best gossip buddy knows all about whatever. I won't get into detail, but I had a situation where my girl was being distant and against my better judgement I snooped and found that she was doing things she shouldn't have been. I am not saying that is happening with your girl, I am just saying that if you feel like something is up and you don't have a history of being overly jealous or suspicious. Then you are probably right.
The thing is, that most people don't go into a relationship knowing that this person is the one. They date around, things are nice, things are fun. The newness of the relationship fills any holes that might show later. Then after 1 to 2 years, you realize...oh shit. If I don't think about this...I am going to wake up 10 years later, I am going to be old..no one will want me...etc. How will I know this is who I was supposed to be with all this time? So they go into this frantic craze, realize that they are in their prime and some of them jump ship.
My suggestion to you is to stop looking at it as a "do i really want to deal with getting another gf or etc" and look at it in the "do I really love her and want to stay with her for the rest of my life"
There is absolutely nothing wrong with being single...and without all details I cannot make a truly informed decision. But it looks to me as if you really should sit down with her in a comfortable calm setting and either decide TOGETHER to work on things, or end it. Just don't get heated up or argue over stupid things.
Oh, if you do end it...find a girl that smokes. Or ideally find a girl that shares your level of smoking. And if you grow make sure any new girls that might be in your life don't know about it. Some girls can do a full 180 and go psychobitch on you and get you busted.
I should stay away from this forum while high.