A question I get asked frequently is, "How do you know whether to stay and fix a relationship or just let go and move on?" Unfortunately, like in most things relational, the answer is, "It depends." Not much initial help, I know, but there are always clues that can help you answer this question.
First, you have to determine the reasons for staying and trying to make things better. How much do you truly have invested in terms of time and emotion? How compatible are the two of you in terms of goals, values, life style, and future desires? Are you putting in more than you are getting back? Do you want it to work because all your friends are paired up, you feel like you're running out of time, or you're tired of being alone? Be honest with yourself now because the truth will come out in the long run.
If any of the hard reasons are present--addiction, abuse, personality disorders--leaving may be the only healthy option you have. Soft reasons--poor communication, growing apart, not "in love"--can possibly be resolved. In addition, if you have children together, you won't be able to truly end the relationship so it may be worth the effort to improve it. The same is true if your lives are entwined in other ways, sharing property, owning a business together, etc.
If you decide to stay and try to improve things, there are a few things you need to focus on. The most important one is to recognize that the only person you have any control over is yourself. You, and only you, are responsible for your choices and behavior. The biggest problem in trying to fix a relationship is focusing on your partner. If you do that, your chance of success will be minimal. After all, haven't you already tried that?
To really fix a relationship you have to put effort into your side of the equation. Relationships are reciprocal and are a function of what you each bring to the table. Being comfortable in your own skin means you have identified and are willing to honor your own needs and boundaries. You can't be part of a healthy relationship if you are not clear on what these are. Settling for less than you need or deserve will only lead to your feeling hurt and resentful. Harboring these emotions influences how loving a partner you can be. Identifying the source AND your part in allowing that behavior to continue is critical if you are going to make things better. A relationship that doesn't work for you is one that isn't going to work ever.
Second, try to look at yourself through your partner's eyes. What statements have they made about the relationship? What requests have they made for it to be different? While their view may be an exaggeration, there is some valuable information in their position if you can see it. Take what you can acknowledge and see where you might be willing and able to do things differently.