It is possible to lose your 'groove' and it is possible to get it back. Let's make this about me!
I spent my university years and the flat-sharing years after them as a fairly popular person in many ways. Not just pulling. Admittedly it was a promiscuous time but also a time when people were fairly but not painfully honest to one's face, especially when stoned, so I knew that I was not overly attractive but thought of as loyal, funny, sociable and a lot of positives that mitigated the fact that I wasn't everybody's physical cup of tea. I wasn't nobody's.
As my friends paired up more permanently during our mid twenties I was increasingly alone. I ended up in a bed-sit rather than in the shambolic but sociable bottle-party house-shares and work got harder and people seemed to get more shallow. I basically shut down my sex- and social life for years on end, drank too much and told myself that it really didn't matter, that I was self-contained. The cat that walks by itself. Really my confidence was shattered. It wasn't just getting off with people, it was talking to somebody from when work finished on Friday night to again on Monday morning.
There wasn't a magic formula for getting back into the swing of things. Perhaps once the first wave of settlers down had paired off the more scarred 30-somethings took their sweet time to network with each other. Perhaps it was doing slightly more age-appropriate things like hobbies and going to places like Davy's Wine bars where other slightly older people met. And not being maudlin pissed must have helped. Who knows?
Whatever it was, another stage began. More civilized, and more expensive, than prolonging late adolescence (but without any mates in either sense of the word by then) well into my late twenties. I seemed to be bumping into more people and things picked up, in terms of a second set of friends who I hadn't met at University or first jobs and other unattached people seemed to around again, some of them freed of relationships they'd embarked on when they were in their early twenties.
I'd say that the traditional male gender role leads to some men finding the process extremely high-pressure, to the point where just avoiding it altogether actually feels better
It is hard work creating relationships, but I don't completely agree that 'shutting down' is either totally a bad idea or totally a good idea. The danger is that the wound-licking stage can become permanent.
(I have never said any of this stuff to anybody, so take the piss carefully, please)