Fact. There is not enough magic in this world to cure a broken heart. Magic gave you an illusion, a pain-killer, if you will; some illnesses cannot be even touched by the glamour.
Neither time could be the solution since some matters are so deeply rooted in your heart that they are eternally in you.
Magnus Bane was aware of all of this, and he was also aware of the fact that the desperation he was feeling in that moment was momentary, as well. He had more than two centuries to learn how to manage himself when he was feeling that low, and a great lot of experience on how to hide to give himself some time to think.
However, since Alec Lightwood had brought his pretty dark head in the warlock's life, something changed. Even when Magnus tried to hide his sadness away, Alec had this little (annoying) sensor that told him that something was off. At the beginning of their relationship, Magnus accepted his boyfriend's company in silence, basking in his light to remind himself to find his own. From a moment on, though, he started opening up to Alec; little by little, fact by fact, he started to rely on Alec, even just to take the burden off his shoulders for a while. This ended, sometimes, in Magnus crying in Alec's arms, with latter brushing his boyfriend's hair, feeling powerless. Once, though, after crying his heart out, Magnus managed to talk to his boyfriend.
"I'm sorry, Alec."
"Sorry for what?"
"I've been a burden to you, lately. I should be the one being there for you" Magnus answered, bitterly.
"Magnus, there's no way in hell I'm not going be there for you when you need me. You could even cry blood and bleed all over my clothes, for all I care. I want to be there for you."
"Why am I not surprised that you are very willing to smear your clothes with blood?"
"Oh, shut up."
Hence, from that moment on, Magnus gave himself a chance to express his sadness and fears to Alec, who always listened carefully. And when Magnus told him that he felt worthless and drained, Alec took him to bed, tucked him in and said: "I don't know how to change your mind about yourself; but, please, know that you are very much cared for, and that I love you very much. It is not easy, but it is important that you know that this sadness will eventually end. As long as you remember to breathe." Cue to more tears by both parties.
Magnus knew that his sadness was not going away anytime soon, but he knew he had Alec by his side. He knew he could fight through it.
And that, my friend, could be enough.