A good way to know if you really, truly belong with the person that you're with is by two things. First, try to imagine your life without the person, and see how you'd feel. Would you be happier without them or more miserable? The second thing would be to imagine being single again, and in that single environment, you're free to do what you choose. In that freedom and single life, imagine if you saw the person that you're with right now at a single's event, function, or wherever, would you date them all over again? In other words, is the fire still alive? You should weigh the pros and the cons in being with someone, and if you have to question staying with them to begin with, you're likely having issues, or second doubts about the relationship. Those doubts should be addressed, but you should also ask yourself those two things.
People always talk about things being greener on the other side, and what it would be like if they were single again. Could they find a better match? Would they be happier, and feel more fulfilled in their relationship if it was with someone else? Well, the grass isn't greener on the other side, but every relationship, and every person in this world that you experience relationships with will be different from the next. Every relationship will have positives and negatives in it, and no situation will ever be perfect. You have to weigh out the negatives and positives and make the best decision for you and your partner, if you're having doubts.
Why are you having doubts? Are you smiling enough in your relationship, and are you feeling happy more than not? Are you willing to work on things in your relationship with your partner, and are they willing to work on the relationship as well? If you both love each other, and you're both willing to improve on things in your relationship, then you should try to do that, and way before your doubts start taking over. You should never foolishly make a harsh, drastic, and impulsive decision to end things, before knowing if you should stick it out, and before trying everything you can to make things work.
If you're in a predominantly healthy relationship together, you should try and work things out, as long as the fire, chemistry, passion, and love is still there. Sometimes you literally have to imagine losing something, to know what it would feel like not to have that thing, or person in your life anymore. You don't necessarily have to make a bad decision, end things, or act impulsively before knowing for sure how you feel. You can simply imagine what it would "feel like" not to have their love and energy around you anymore. It's one thing to imagine not being with someone anymore, and recognize the feeling of loneliness. But, it's another thing to recognize the feeling that you get without someone, and realize how much that you'd long for the person to be back in your life again. I don't believe in breaking up, and making up again. In other words, if you break up with someone, you should be sure about it, because break ups should be final, at least for the most part.
Remember to ask yourself three things if you're debating leaving the situation that you're in. First, ask yourself if you'll be lonely without the person, or if you'll be in sincere heartache from the person's absence, and long for them. Secondly, ask yourself, once you put yourself out there in the dating scene, would you end up searching for the same type of person? As well, if you saw the person that you're with now on the market, would you feel the desire to go back with them? Finally, figure out why you're having doubts to begin with. Find out if your partner is willing to work on things in the relationship, and figure out if you're willing to put in the effort, and work on things to improve your relationship as well.
Remember, it's not greener on the other side, it's just another shade of yellow. Every situation will have positives and negatives, and you should never expect to find perfection in a person, or in a relationship. If you truly love someone, then you'll be willing to put in the effort and make things better for the both of you. However, you both have to put in the effort equally to improve your relationship. You can't expect one person to do all of the work in a relationship. Having said that, if one of the two people in a relationship feels that the "other person" is the one that needs to work on themselves solely, then your relationship has a lot more problems than you might think. If your situation sounds something like that, you should take a step back, and reevaluate the decision to stay with this person, because relationships take work, and effort from both people.
We all know and have heard that there are plenty of fish in the sea, and that there is no shortage of possible future matches for any of us. It doesn't matter how attractive you are, how old you are, how prestigious your job is, etc., because there is someone for everyone in this world. When you're in a relationship, and you're having doubts about it, you need to figure out whether or not you can imagine staying with this person for the long term. Think about it, will you stay with this person if everything gets better, healthier, and if both of you are willing to work on the things that one, or both of you is concerned about? Do you think that you'd feel happier then? If not, it's likely time to reevaluate your relationship, and possibly end things sooner, rather than later.