If I recall correctly, you're exercising a lot and your clothes size is going down. You're probably building muscle, and muscle weighs more than fat. So you're losing fat and gaining muscle, hence your weight either goes up or stays the same, but it's not traditional "weight gain" in the sense that your body is storing fat. Your body is just storing muscle instead. For example, some bodybuilders are 300+ lbs and they would normally be considered "obese" for their height, but they have minimal body fat, so they're actually not obese. Yet, someone who is the same height as a 300 lb body builder but weighs 200 lbs could easily be considered obese because of their body fat percentage.

But anyway, even if Seroquel is legitimately making you store fat, there are ways (besides Metformin) to prevent (or at least mitigate) that side effect. I'm not saying you SHOULDN'T try Metformin, but I'm just giving examples of what you could do in addition to or besides Metformin:

Try reading this:
7 habits to help you lose weight and keep it off - Harvard Health

Also, while Seroquel and Seroquel XR did NOT make me gain weight, it did increase my appetite. I counteracted the effect by drinking more water to quell the hunger levels. I also exercised before every meal (at the time I took it). I work a 9-7 job and I'm able to fit in exercise in the middle of the day at work if I really want to. I just take a walk outside, making circles around the outside of the building.

There are lots of indoor exercises you can do if you don't want to workout outside. You can find some awesome stuff on YouTube, especially quiet indoor exercises that don't bother your neighbors if you live in an apartment building (like me). So do about 10-15 mins of those exercises before a meal and that should help reduce hunger -- I promise. Even if you go out to Starbucks to eat food or drink coffee, you can still exercise beforehand. OR you can exercise afterwards if you don't have a food coma.