That's only a 2" topper. And it's memory foam at that. You are very likely to sink through all the way to the bottom. No wonder you feel uncomfortable. We don't even need to ask any more details (e.g. BMI, height, weight) to tell you that this is not a great option.

Also, melatonin isn't going to do shit in this situation. Your body naturally produces melatonin around bedtime. That's what makes you realize that it is time to go to bed and helps with falling asleep. It wears off relatively quickly and it does nothing to keep you asleep.

You might naïvely think to take more melatonin, but that's just going to really screw up your sleep pattern. It can help with forcing your body to synchronize with the time of day. But if you take it at other times of day, it just confuses your daily rhythm. It should only ever be taken at the exact same time.

Now, for your mattress, you will need to find something that goes under the very thin and soft topper that you are sleeping on. IKEA does have a few options. But I'd recommend heading to Costco instead, as they have a much better return policy. If you don't have a membership, ask around; one of your friends or family is bound to have a membership.

With Costco, unlike pretty much any other store, I don't think there is a time limit on returns of mattresses, but you might want to double-check with customer care.

If you are tight on money, I'd recommend picking something like https://www.costco.com/novaform-overnight-recovery-3%22-mattress-topper.product.100476301.html and putting it under your existing topper. It's considerably firmer than what you have right now, and will provide some degree of support. At 3" it is thicker than your current topper, but still relatively thin as far as a full mattress would be concerned.

This is not going to be a perfect solution. But it will be a huge improvement over what you have right now. It'll almost feel like a real bed :-) When you have more money saved up, you can either buy additional (relatively inexpensive) components to turn all of these parts into a full mattress. Or you can return the 3" foam topper to Costco and buy a new mattress. They do have a couple of good and relatively affordable options.

Personally, I wouldn't buy a Tempurpedic, if money is tight. You pay a lot for the brand name, but you can probably get something better for less money. Read this sub for a while to learn more about what to look for in a good mattress.

Also, just to make this absolutely clear, don't even think of buying additional memory foam. The whole idea behind memory foam is that it lets you sink in and cradles your body. That's nice for a top layer, but it doesn't work at all to provide the overall support that you need. If you stack more memory foam, you are not actually making a usable mattress and you'll still feel miserable in the morning -- assuming you even manage to sleep through the night.