I've found a lot of times that when I fixate on thoughts, that I have to make myself a distraction. Whether that's starting a movie or reading something, or calling someone and asking how they're doing to focus on something else, it helps me. Sometimes also I clean around the house, or try to be productive and find that finishing a task makes me feel better (I'm also a clean freak, so cleaning is calming for me-I know it's the opposite for some).
All medicines have potential side effects, and affect people differently, so maybe one of your medicines did cause this. The important thing is finding a solution. If being off the singulair doesn't help, or changing the dose of the paxil doesn't change things, I'd go back to the dr and talk to them again. Of course, with all depression/anxiety meds, you have to give them time to help you.
Hopefully changing your meds will help you feel back to your normal self soon. Let us know how you continue to do.
31 yr old female-dx with Crohn's in '97 after emergency resection and appendectomy, 2nd resection '05
Currently on Humira, Prilosec, Effexor, Seroquel, Calcium, Vit D, sublingual B12; phenergan, ultram, clonazepam as needed