In terms of how I approach the issue of a client who would like to have a relationship but are finding it difficult to get into lasting relationships is to look at how the client is 'being' as opposed to what they are 'doing'. Often, there can be an issue where our impact on others is not what we intend - so people can find they come across to others as intimidating or distant or not interested in them.
Sometimes this can be because the client finds it difficult to access or express vulnerability - if we are used to being very independent (which is a great quality!) it can be difficult to experience vulnerability which is essential if we want to be in intimate relationship with others. So as a coach, I use an approach called 'polarity coaching' which helps client work through common polarities such as 'strength vs vulnerability' or 'independence vs dependence'. This is particularly useful if we find ourselves drawn or strongly identifying with one particular pole - such as strength - to the extent that we reject or don't like to experience the other pole.
Another approach can be to look at our reactive patterns - in terms of relationships if we have a pattern of pulling away or putting up barriers to others as a protective mechanism (mainly we do this without realizing, unconsciously) we can start to observe this and start to look at what might be different if we are more able to be more centered and open to others. I use a technique called Leadership Embodiment which uses gentle physical movements to examine our reactive patterns and then shift into a more centered and open state which can be more welcoming of other into our lives.
And one other approach is to look at our saboteur's - we each have ways that we sabotage ourselves. Some people get anxious with relationships and find themselves becoming too clingy and 'needy'. For others the saboteur might make us choose people who are unavailable. For others the Saboteur might be making us give off cold and unwelcoming vibes. It might be useful for you to look at how your saboteur might be sabotaging your desire to have a whole-hearted relationship.
No one can pinpoint or know exactly why you haven't been successful but it's in the process of working together that you and the coach will start to see the 'road map' of what's getting in the way of you having a successful relationship. It is a matter of looking more at who you are being - and then playing around with different ways of being, even if at first it is uncomfortable, to see if you find yourself doing different things or having different results.