OP, I'll offer a slightly different perspective, in that I used to really have commitment issues, and general insecurities around relationships, from my own family background - so I think you need to tackle that first.
BUT...as well as believing that commitment issues might be holding you back, I think it's also possible that you are not with the right person. I am in a 'perfect' relationship - it's not perfect, but he is perfect for me and I know that after years of being with the wrong people I can suddenly see what that means...it's like a lightbulb 'ahah...THIS is what love feels like..'
After a year of therapy sorting out all my own issues, I found 'the one' - and I am afraid I can confirm that real true love does exist! Maybe it's easier sometimes to believe it doesn't, so that we can stay in unhappy situations.but real love is something you know when you have it. That doesn't mean AT ALL that it's all a bed of roses, but it does mean genuine joy, love and pleasure in each others companies -whatever the ups and downs of daily life - and the safe knowledge that you are with the right person for you. Yes, there are lots of people we can be happy with - I don't think that means that finding 'the right one' isn't possible - but being happy and content is possible.
The first step for you is to work out how much of your disatisfaction is down to your own insecurities and unhappiness/ commitment issues - if you are hankering over lost love, that is a sign of low self confidence and anxiety around relationship choices - let it go, it's over, if it was right it would have worked out. He's a daydream, not a reality now.
You can acknowledge your unhappiness in this relationship as real - and pick apart which parts come from your own issues and which are genuine imcompatibilities.
I wouldn't recommend Relate, I think you need a proper trained psychotherapist not just a counsellor - if you are going to be looking into your past, which you will have to. You can get cheap deals - look on the BACP website..honestly, it's the most life changing thing - and will help you get to the bottom of hwo you feel about your partner and untangle all your guilt etc.....