The problem with "good" relationships after "bad" ones is that you find it very difficult to believe they are for real. Most times you try to find reasons why the new one won't work simply because you want to save yourself the pain of going through yet another bad break-up. In my personal experience (and I have plenty - being married to an abusive husband and then an alcoholic, not to mention becomming involved with an abusive alcoholic after both of those) defining a relationship in terms of good and bad is never a good idea. Just because it seems better than the ones you have had in the past does not make it a good relationship. Each time I had a new relationsip I thought it was great because it did not have the bad qualities the past one did - it turned out that it just had OTHER not-so-good qualities that it took me quite some time to see because I was busy trying to focus on the good that I saw.
9 times out of 10 the novelty will wear off and the guy you are with will be not so perfect afterall.
In any case, if you are having these feelings of worthlessness then perhaps it is not time for a new serious relationship just yet for you. You cannot love someone else until you love yourself or at least take the time to work on the problems that make you dislike yourself so much. So if he is in fact a great guy, it might not be fair to expect him to be with someone who cannot possibly love him back.
Just a note...humans are incapable of unconditional love.