"Happy heavenly birthday Zen. At the root of all the emotions I have today there is insurmountable love," she captioned a June 23 Instagram video of precious moments she spent with her baby before he died Dec. 5. "I always try my best to remain positive but right now I can't help but cry out ‘it's not suppose to be like this.'"
Alyssa went on to say that she can imagine Zen "smashing his cake and crawling around." Ultimately, reality set in and she grieved how she'll "be the one to blow out his first candle."
"I will wish he was still here with us," she added. "GO REST HIGH."
On the Dec. 7 episode of Nick's former eponymous talk show, he shared that Zen had passed away after a battle with a rare brain cancer, which the duo learned after taking Zen in for a routine doctor's visit.
"And I think it was called, if I'm not mistaken, fluid that was building up in his head, and that was the cause," Nick shared, referring to the condition known as hydrocephalus. "His head was starting to get big. When we found out it was more, they called it a malignant tumor in his head. Immediately, we had to have surgery. Brain surgery."
Two months after Zen's death, Alyssa opened up about her final moments with her son, "I know everything around me would stand still. It would be just you and me," she wrote on Instagram in February. "My last words to you were 'I'm here, I love you.' That will ring true until the end of time."