Feeling unworthy?! This can be quite simple; anytime anyone deems themselves unworthy for a relationship, or for someone, there are two scenarios that could be at play; you feel unworthy for someone and are afraid to approach them to try and create a relationship with them, or you feel unworthy to stay with someone.
If you feel unworthy to approach someone, this is a common feeling through insecurity. This situation would be, "I want to ask him/her out, but I don't think they'd like me, I'm not good enough" etc. Something like that. If this is the situation (which is very common) it is something you can only build confidence to conquer your insecurity. There isn't much advice I can give for this, although I can explain WHY you might feel this. You cannot control how other people feel, or know exactly how other people feel. Because of this unknown factor and the lack of control, self is first to blame. You can only blame yourself because you have no control over the other person. You may also not have much experience in relationships and feel inexperienced in comparison to that person whom you adore. There are several other insecurities and lines of denial that lead to self-blame, but you probably don't actually care to hear it :^P So, the only advice for this is to... GO TALK TO THEM DAMMIT!
If you feel unworthy to continue being with a person, this is irrationale. Commonly, this occurs with long distance relationships, with both genders. This means that, if you are feeling unworthy to continue being with them, you may be concerned about the future. If the future bothers you, and you have no control over it, and you feel that you must end the relationship because of it, that is entirely up to you; for you may be avoiding hurt and asking for commitment over long periods of time with minimal communication is just that, asking for commitment. This puts the other person into a tense spot; for they must decide wether or not they can commit to you (you also must decide this, thus your worry as of now). This is something you must confront with your other and decide together, for it is your mutual feelings that will influence your future.
If I just went onto a tangent, and it didn't apply to you whatsoever, and you feel that your other person should be with someone else, and not you; then I must laugh. They are currently with you, because they want to be with you. If you think they should be with someone else because you don't feel good enough for them, you're being irrational and cause problems for yourself. You are also searching for attention however (this is good to read if your other is thinking this). This is commonly a hunt for attention and support, which most likely means you have fallen for this person and love hearing their compliments about you. This can be a very annoying and selfish thing to do to someone for it focuses on you and causes agitation for your other which nobody likes to deal with. Most people who do this, however, will deny that they are searching for attention and continue on the guilt trip to try and "milk" as much compliments as possible. If they continue to do this after you have asked them to stop then there is only one thing you can do; everytime they try to tell you that they feel unworthy for you, change conversation, and it is better to tell them that you want to change conversation, because you feel that it is a dumb thing to talk about, because you know how you feel and they shouldn't question it. Everytime they start to feel down and feeling unworthy about you two, basically, tell them to shut up and change the subject.