Mal was his needle, dangerous and lethal.
She was the poison that kept injecting itself into his life, regardless of how hard Cobb tried to rid himself of her. Like a snake, she slithered around his skull, leaving her deadly mark wherever she went, assuring and reminding him that she was ever present in his subconscious, waiting to strike.
In a very literal sense, she was his drug; a necessary evil to keep him functioning even as it slowly widdled away at his sanity. There was no way he could keep living with her, but to exist without her meant death.
His friends worried about him, encouraged him to give her up even if they didn't understand the full affects of what Mal was doing to him. And yet, Cobb couldn't give her up. There was something terrible and sweet about her that kept him coming back to her fake, warm embrace.
Mal was addicting.
How could anyone give up the familiar comforts of someone they'd used to love? When Mal died, he was crushed. The overwhelming amount of guilt he'd felt surged through him and left him spiraling into a dark depression. These feelings he felt made the illusion he created all the more desirable- believable.
When the projection of her had first manifested itself within his mind, he'd been elated, positively thrilled. It was like he was being given a second chance, and he encouraged it to keep coming back every chance he could. Like every other drug addict, he fell into its secure trap. Cobb failed to realize how badly it was affecting his psyche until she began interfering with his day-to-day dealings, sabotaging his jobs.
Cobb realized too late that he'd become far too dependent on her, allowing this fake Mal to grow strong and spread her roots deep into the depths of his subconscious. His projection of her had grown a will of its own, refusing to be controlled or sent away, going so far as to kill anyone who step foot in her realm. Even then, Cobb wasn't sure he wanted to get rid of her. She was supportive, in her own right.
However, he soon grew ashamed of her. Now that she'd grown out of his control, he was no longer able to hide what he'd been using her for. His dependency for her began to cause problems for him and his teammates, and suddenly this wasn't a private matter anymore. Still Cobb refused to do anything about it, much to the disappointment of everyone around him.
This projection, this false image of her was all he had left of Mal. Cobb loved this woman, and even if she only existed in the dregs of his mind, to him she was as good as real. This Mal managed to ease and swell his guilt all at once, comforting and distorting his perception of how things had been.
She was his needle and helped him learn to cope with the damage done. It was unhealthy in all regards, but like every junkie knew, these things, when left untended, could not be undone, and destroyed things in the most ugliest of fashions.