You have nutrient lockout. Miracle grow was probably already too 'hot' for seedlings, and then you added more fertilizer. Too much nutrient build up in the grow medium can make the nutrients inaccessible to your plant. It isn't a nitrogen deficiency, if it was then you should expect to see the new growth losing colour before the old growth. Yellowing on the lower leaves strongly suggests nutrient lockout to me, especially since you have added fertilizer to soil which already has nutrients in it.
Take 6 gallons of plain water, use this to water your plant in a single watering so that most of the water runs off. This is known as 'flushing'. I think using almost four times as much plain water as the volume of your grow medium should be sufficient to get rid of the nutrients in your soil. Go to a store which sells gardening products, this could be a nursery, certain hardware stores, or a hydro store. Buy some nutrients which are designed for use in the vegetative stage of plant growth - when you check the NPK ratio there should be considerably more nitrogen than phosphorus or potassium. I would be looking for an NPK ratio along the lines of 3,1,1 or maybe 3,1,2. Use liquid nutrients, not slow release pellets. The label will tell you how much to use - don't use as much as the label says - on the first feed you should use 1/3 of the recommended dose, if that goes well then on the second feed you might up the dose to 1/2 or 2/3 of the recommended dose, by the third or fourth feeding you should be using the recommended dose. When you hit the recommended dose, just stay there unless you start to see symptoms of nutrient deficiency (again, this will be on the new growth at the top of the plant), in which case you should increase the dose. I expect that you wont presently need to water your plant much more often than once per week, unless you live in a fairly hot/sunny area - by the time a few weeks pass your plant should be able to handle the recommended dose. If you need to water your plant much more frequently than I have assumed then you might want to increase the nutrient dose more slowly than I have suggested. (The word 'need' is important, I expect you are watering your plant more frequently than once per week, but I doubt that you should be given the age of your plant and the relatively large pot it is in.)
Don't top your plant now. You will just stress it out. I highly recommend that you look into topping and training your plant down the track. But, while you are trying to solve problems on a seedling the last thing you want to do is add additional stress. Even if your plant had no issues whatsoever, I think that it is a bit too young and underdeveloped to top.
The yellowing single bladed leaves in your photo are 'true' leaves. They should die long before you harvest your plant, but it is not normal for them to be yellowing already. Follow my advice and I am quite confident that you will be able to rehabilitate your plant and get it healthy. Given that you only have one plant, I hope that you have used feminized seeds, because you are going to be super upset come flower if it turns out to be a male.
The plant should be perfectly fine for two days without water. As a beginner grower, you are much more likely to over water your plant than to under water it. I guarantee that if you saturate an almost two gallon pot this will be all that a two week old plant needs for 5-7 days easily. It is unfortunate that you gave the plant more nutrients, because your problem is too many nutrients, not a lack of them. Flush the plant and then give it conservative doses of a nitrogen heavy nutrient solution.