I haven't clung since my early 20s. At that time a dear, dear friend of mine did a 180 on me that I wasn't expecting. In shock, I kept trying to talk to her when she was giving me every message subtle and not-so-subtle that our friendship was over. It was a life lesson for me.
That's not to say I still don't cling. Mentally I do yes. Friends and partners are so precious and few for me. If I've not heard from someone I have great amounts of love for, my instinct is to create some light and warmth where this is none at the moment. In the past, that would result in me reaching out a lot. In this day and age though (I'm 35), I'm seeking to balance contact vs. reciprocation. If I catch myself engaging in clingy thoughts - I try to distract myself with a good read, perhaps something academic or some random musical adventure equivilent to a ride on a winding mountain road.
I've learned the intensity of my internal emotions I have to keep them "managable." That is to say, how much I send out into the world. It's hard because I don't hold them in well at all. I'm almost in a steady tightrope act to keep things "ok" for 'out there.' Last thing I need is my coworkers looking at me like I'm from Proxima Centauri. They sometimes do anyway, because I work in a highly extroverted field.
But you get my gyst... (I hope)