While I would like to say you could look into something to lower thyroid activity, it's a dangerous practice. Overall, as you get older you will be grateful for a high metabolism to keep fat levels down.
But looking at your pics, I have a different opinion. Being a hardgainer is your body type/genetic blueprint and it not simply a metabolism issue. I can tell you from my experience, starting out the same, you simply need to eat eat eat - the right stuff - and stick with workouts aimed for hardgainer bodies. That means short, intense workouts with relatively heavy weights. Avoid over-training (the most common mistake). No 2 hour sessions in the gym. No innumerable variations of exercises for the same body part. Do 1 or maybe 2 body parts per session. Do 2 or 3 exercises (after warmups) maximum per body part and move on. Get plenty of protein. Don't be afraid of carbs - but focus on low-glycemic carbs. A good bodybuilder diet is 40% protein, 40% carbs, 20% healthy fats. You need to take in more calories then you expend, so try to calculate how many calories you need in a day and then divide by 5 or 6 meals a day, including shakes. Stay away from those 1000 calorie weight gain shakes. The carb/protein balance is way out of whack and you'll get fat from them. Use up to 60g of glutamine a day for muscle repair and recuperation. Use R-ALA to stabilize your blood sugar, which minimizes fat deposition, among other things. Take a supplement like a standardized tribulus/fenugreek blend or fadogia agrestis to boost your natural test production. Tribulus is easier to find, fadogia works better.