I've written in the past about feelings of inadequacy or self-loathing, especially as they pertain to relationships. A person who doesn't feel good enough for his or her partner may experience anxiety that endangers the relationship, and in some cases the anticipation of this anxiety may be enough to make that person avoid relationships altogether. If you struggle with these feelings, one thing you may want to do is this: rather than focus on whether you're good enough for the person you're with, focus on whether you're right for that person.
How are these different? Let me draw an analogy to ethics, where the distinction between the good and the right is fairly well drawn (if not precise). Choosing an action to maximize the amount of goodness (such as utility or happiness), as consequentialists do, involves tremendous measurement problems. They must predict, calculate, and compare the good done by each alternative course of action, making sure to account for uncertainty, unintended effects, and so forth. This is literally an impossible task, as not every contingency can be anticipated nor its impact calculated. This isn't a slam-bang argument against consequentialism, of course, but it does seriously impair its practicality.
In much the same way, deciding whether you're good enough for someone involves lumping all of your good and bad qualities together and somehow rendering them comparable, so you can "add them up" and assess whether you measure up to the other person. Of course, we don't really do that, just as utilitarians don't literally calculate and compare different options (well, except the bureaucratic types). But we do, somehow, take measure of ourselves, even if it's just a feeling rather than the result of a detailed algorithm.
This exercise is often doomed to fail, however, because when we're enamored with people—so much so that we're wondering if we're good enough for them—we idealize them, seeing them as close to perfection as someone can get. But the more we do this, the less likely we are to compare well to them. And if you have self-loathing problems, that "calculation" will probably be biased against you, downplaying your good qualities and emphasizing those you see as negative.
That's why it's better to think in terms of being "right" for somebody. None of us is perfect; we all have our quirks and idiosyncracies. And each of us is unique, with our own needs and desires. It doesn't matter how your strengths and weaknesses measure out in the balance; what matters is how they fit with the other person's. In philosophy, the "right" is the focus of deontological ethics (like Immanuel Kant's), in which acts are judged to be right or wrong based on their intrinsic features according to basic standards of justice, equality, or dignity, not some measure of their consequences. In the same way, being right for somebody is not about measuring, calculating, and comparing; it's more general and abstract, aimed at making an intuitive judgment of fit.
Maybe you see the person you're in love with as incredible, accomplished, and confident, and you don't feel that you match up. Your assessment of yourself and the other person aside, you can still feel right for that person if you realize that you provide what he or she needs (and vice versa). Maybe that person has success, but needs someone to share it with, or maybe isn't as confident as he or she seems or would like, and needs you to provide support. Or maybe that person just needs somebody who knows how to make him or her feel appreciated, or safe, or loved. And he or she doesn't care how "good" you are, but only that you're right for him or her—that you fit.
At risk of beating a dead metaphor, recall the jigsaw puzzle of previous posts (here and here): it's not about how big or pretty each piece is in a jigsaw puzzle, but how well the pieces fit. And what I wrote in yet another post applies here too: if that person chose to be with you, you need to trust that he or she seems something in you, very possibly something you don't see in yourself, that fits with him or her. Your partner has decided that you're right for him or her—and by being with that person, you might even find out why.