I was on seroquel at one point too (I'm bipolar). It made me gain weight as well. The thing is, there aren't many psychiatric medications that don't have weight gain as one of the potential side effects. I've been on zyprexa, celexa, lithium, wellbutrin, risperidone, ativan...the list goes on and have gained weight with most of them (celexa and zyprexa being the worst for me). Your trainer is right, you're going to have to work harder than other people to get the weight off but just think about how great it will feel to know that you've done it. And the main thing you need to be concerned about with regards to your medication is whether or not it is working for your mental health concerns. For me, I believe that if it is its worth being on because I'd rather be fat and sane than thin and crazy.
Good luck with your weight loss! I wish you all the best.