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The thing to understand about men is that they can separate sex and affection easily. They also enjoy challenges, which is why being "too nice" is one of the top ways women de-value themselves in men's eyes.
You seem to think that because he flirts and sleeps with you, there's a relationship or that he has feelings for you. That's a woman's definition, but not a man's.
He doesn't leave you alone because you give him sex, and men will take that when offered. However, it doesn't indicate anything about his feelings. You state you give your all when in a relationship, but it sounds like there was no acknowledged relationship here, just some flirting and sex.
It's important for women not to sleep with men before they know if there's a true relationship, as the hormone oxytocin is released in women when they have sex, and it causes them to bond to the man, even if he's not appropriate for them. Here's a couple of articles that explain this phenomenon:
behind falling in love
Going forward, it's important for you to get some information about how the male mind works.
Here are two books you should read:
The first is Temptations of the Single Girl http://www.amazon.co.uk/Temptations-Single-Girl-Nina-Atwood/dp/1587368986/ref=sr_1_1?s=books&ie=UTF8&qid=1323090483&sr=1-1 This
book should be required reading for women before they start to date. The
"temptations" are thinking that we can change a man or his behavior, settling
for a man who doesn't want a commitment when we really want marriage or a serious relationship, and wasting time with the wrong man because we're afraid to be alone.
The second is Why Men Love Bitches: From Doormat to Dreamgirl - A Woman's
Guide to Holding Her Own in a Relationship. http://www.amazon.co.uk/Why-Men-Love-Bitches-Relationship/dp/1580627560/ref=sr_1_1?s=books&ie=UTF8&qid=1323090405&sr=1-1
Reading and taking to heart the information in these books will save you a lot of heartache in your dating life.
It is absolutely time to "move on" from this guy. He's given you no indication of interest, and has told you why he's not interested. It doesn't get any clearer than that. And if your emotional energy is tied up with him, you'll not be likely to meet someone new. Read the books, realize that men only value what they have to work for, and put your attention to the future and a man you'll meet who can't wait to spend time with you, and who's ready to commit to a relationship.