Completely anecdotal, but I had similar reactions when taking both Zyrtec and singulair for asthma/allergies, and switching out the ceririzine for loratidine or diphenhydramine(benedryl) got rid of the symptoms. Back when I started on singulair, I had no idea that certain allergy medicines could have an effect on mental health, but nowadays there's a disclaimer packet every time I pick up a prescription of singulair. I had to do some experimenting with different combinations of allergy medications to find what worked best for me without side effects, so I'd recommend having a conversation with your doctor about what would be best for you to try
Can't go wrong with America's Test Kitchen buttercream, here's a video for their chocolate buttercream, starting at 7 minutes. You can use a hand mixer instead of a stand mixer, and their vanilla buttercream is even easier to make