Here is about the most straightforward, nonsarcastic reply that you are going to get:
At 18, there is no way that you have fully tuned your lifting routine at all. You probably don't have your training splits optimized, your training time length at the best level, don't have the lifts that you already do being done correctly, don't have the most effective lifts in your routines, etc.
Your diet no doubt needs some work too. You need to be eating 6-7 times a day, taking in an evenly divided protein amount in each meal. Your protein goal should be at least 1g per pound of bodyweight...AT LEAST. As for you shakes, you need to get one in 45min before you lift and one right after. Your pre-lifting shake needs to be about 60-80g of complex carbs from oats or maltodextrin. It also needs to have a source of good fats(Natural PB) as well as about 30g of whey protein. You can mix it with milk for added flavor and protein. Your post lifting shake needs to have at least 40g of whey protein along with 70-100g of simple sugars, preferably from dextrose. Mix your post shake with water only. You need to get your post-shake in as soon as possible. I put all my dry components in a shaker and mix them with water at the gym as soon as I am done.
Before you go to bed, you should have a meal of at least a cup of cottage cheese. I add a half cup of plain yogurt and a scoop of whey to mine, too.
At 18, I thought I knew what I was doing too. I weighed about 170 at the time and thought I was about as big and strong as I was going to get. I am now 210 and am bigger than I thought I would ever be. I continue to learn more and more about my diet and training regimine. You are never at your limit. Keep in mind: train smarter, not harder.