It is beyond me to describe everything I felt when our son killed himself last summer. But one thing that tormented me was that, although I longed to empty my heart with a flood of tears, I couldn't cry. I'm 60, so perhaps it's just another of those secretions that dry up with age. I hated the way I could contort my face in private into an expression of the agony I felt, with no effect except a slight moistening along my lower lids, and then a second later rearrange my features and turn dry-eyed to the world again as if nothing had happened. It made me feel my grief wasn't real.

So after a week or two I bought a small plastic tube of tiny metallic stars. I tipped the contents out, and began an eccentric bedtime ritual. Every night I wept what little I could into the empty transparent tube. I felt I owed Andrew these tears and I planned that when the tube became full I would pour it on the stump in the garden where he'd sawn down a tree for us a month before his death. It would be my offering to him.

I would sit on the bed holding the tube, and deliberately remember the policewoman coming to the door, my voice asking, "Is he alive?" and her grave, silent stare in reply. That memory was enough to produce the faintest lip of liquid inside the rim of the tube, hardly more than a silver line, but something.

I'd force myself to remember seeing him, lying under the red sheet. He was cold and iron-hard and the shock of realising that he, himself had changed his lovely, warm, breathing body into this terrible thing could coax out a tear. Gradually, what had started as the merest frosting on the inside of the tube started to coalesce into drops.

He had left us no suicide note, but two days after he died in August, we collected his belongings from his home and found a box of letters to his ex-girlfriend. In his small, neat handwriting, most ended "Loves ya babe!". But one stood out. The one in which he explained that his "little man" wouldn't do what he wanted it to, and her reply, wondering if it was her fault, and what she could do to help.

Shocked, I had phoned her, hoping to hear that things had got better eventually, but she said no. In the four years they had been together they had never had full sex. This I could not bear to think of. How cruel it must have been for them both, how good she had been to stay so long. And poor, poor Andrew. The thought of him trying and failing, and keeping it secret, never having the sense to go to the doctor, was a sharp stab. How could I never have guessed, why did I never ask? Memories of conversations came back, when he was trying to speak of sex, but was blocked by my obvious embarrassment. Why hadn't I realised that his only reason for raising the subject would be that he needed help?

Days later, my sister discovered research on the internet that suggested his impotence could have been caused by a terrible head injury he had suffered when he was seven and fell out of a tree. She discovered that about a third of traumatic brain injury survivors suffer damage to the pituitary gland, which can cause impotence and depression. It is under-diagnosed, but treatable. To us the information was electrifying. We felt that if we had only known, we could have saved him. Also, if he had known there was even a chance that his fall had caused this it would have spurred him to go to a doctor. If I felt so inadequate because I couldn't express my grief, how must he have felt, when he couldn't express his love? I thought of him with all his happiness and confidence destroyed from inside, hiding his despair behind that fatally jaunty facade. Yet in the end he had said nothing.

As the months passed the tear-level mounted slowly, a whole centimetre now, almost reaching the label. Crying seemed to come more easily. It was not just the loss, it was that I'd misunderstood him so harshly. I had been impatient that, five years after his girlfriend had left him, he still hadn't moved on. I had thought him sentimental and over-romantic. I'd wanted him to be tougher, and he must have felt my lack of sympathy. But how could he move on, with this miserable secret? He must have wondered who would accept him.

When I really wanted to cry, I thought about his laptop. The police had taken it away when his body was found in his home and did not release it to us until November. On it, we found his emails, which told two stories. The story we knew, of his busy social life filled with friends' weddings, visits home, digging sessions at the allotment with his friends, the triathlon he had done for work and the kayaking course. And, in parallel, the other story of how he was slowly sucked towards death.

To our dismay, we discovered that he had ordered a suicide manual seven weeks before he died. Ten days later a Paypal entry shows him buying some of the equipment he needed, then some extra bits the week after that. There are the emails to his understanding boss (who knew about his depression when we didn't) saying that he'd had a bad night and didn't feel he could come in. Emails to the counsellor we didn't know he was seeing. An email to his ex-girlfriend, apologising for the depression that had spoiled their relationship. The time-delayed emails he sent on the day he died to the police, his boss, his friends. ("The responsibility, guilt, weakness and shame rests entirely with me. I am who I am and I can't change that.")

I'd review this whole sequence and it would be like watching him slowly falling away from me, as if he were behind glass and me powerless to help him. I do not believe that he completely wanted to die. Why would he have had the counselling? I wanted to smash through the glass to this past Andrew, to take him in my arms and comfort him, to tell him how much I loved and respected him, how brave I knew he had been, say all the things that might have helped if I had said them at the time.

In the meantime I made progress with my tube. The level of liquid rose to almost a third full. But then, one bad night when I couldn't sleep for the barrage of sad thoughts, I felt big fat tears coming. As I flailed to switch on the lamp and grab my tube, I spilled the whole thing down my front. Four months' crying became a wet patch on my pyjamas, roughly over my heart.

I have started again, more cautiously. But I discovered from the optician that some "floaters" I'd developed were probably the result of jabbing a hard object against my eye. I think maybe I shan't be able to fill the tube after all. It has all been a silly game with myself. Andrew doesn't know how I feel, he doesn't know how old my face has become, how my left frown-line has deepened since August, how his father and sisters and friends miss him, how his empty house that he was once so proud of grows daily danker, darker and more desolate.

The only thing that has given me any comfort since is a dream. I had gone in to where Andrew was sleeping with his face turned away, to try to wake him up. I sat on his bed and stroked his warm head. Afterwards, even when I woke up, I had the feeling of his hair under my hand, and his hard head below, still ridged from where it was fractured 24 years ago.

• All names have been changed. For information on brain injuries, call the freephone Headway helpline 0808 800 2244, or visit headway.org.uk.

• Do you have a story to tell about your life? Email it to my.story@theguardian.com. If possible, include a phone number.