Hi there, do hope you get the medicine for your mental health sorted out quickly. I am sure there will be other suitable medications that you can use with metformin if they prescribe the latter for your glucose intolerance.
Like Sid, I had to have life saving steroids in the past and they can lead to diabetes. Being finally diagnosed can be a relief as you have a name for all those horrible symptoms and you can then do something to control them by adjusting your diet and lifestyle.
I have just taken a look at the contra indications for your current medication and it does say notify your doctor if you have high blood sugar or the symptoms of diabetes because diabetes complications can be serious. I notice it also states that you must not become dehydrated when on seroquel. My leaflet on metformin also states that. That could be the one problem but you can aim to keep hydrated and once the raised blood sugar symptoms are better controlled, excess thirst and perspiration or feeling hot should be lessened if you suffer those. You current drug does have the side affect of upset stomach and metformin can do this the first few days you take it. For this reason, your doc might want to find more suitable medication if going on metformin is right for you and will prevent full blown diabetes from developing.