It may be that the fertiliser has burned the roots.

Sandy soil is 'hungry' and needs bulk amounts of compost. You don't have to do the whole garden at once. For the tomatoes, dig out a big hole for each plant - about 18" deep and wide - and have a two gallon bucket of good compost (ancient enough so you can't tell what's in it) for every plant you put in. When you plant your little seedling tomato - and its stake, you put back a combination of the natural soil and the compost. Water each plant in - don't press it down. (That helps the air to get to the roots and they need to breathe.)

When you want to feed your plants - water the ground around them FIRST. This is essential if you are using a concentrated feed - even organics. For liquid feeds - try halving the recommended amount. More is rarely 'better' when it comes to plant feeding - unless you are feeding the actual soil with heaps of compost BEFORE planting.

You might want to try using slow release pellets around some plants - just to see if it works well for you in your situation.

No matter what it says on the label - you still need to 'find out for yourself' what works at your place in different weather and seasonal conditions. (Gardening is a lot like bread-making...every batch of flour is different and changes the results.)

What can you do?

Start afresh in a different patch of ground. Put some compost in the area where things have curled up and plant some cheerful annuals such as marigolds to keep the nematodes at bay. Water deeply and add mulch to help the soil and plants stay cooler over summer.

If it wasn't fertiliser burn - dig up with care and check for bugs underground. Squash what you find and - pile on the compost (because it helps most plants to resist attacks by pests.)

As a sandy soil owner - if you get the habit of planting each plant every time with a half to two buckets of compost (depending on size and if it's an annual or long-lived plant) you can build up your soil. It's what you do to get the great benefits of having a quick-to-warm and free-draining soil.

Hope that helps.