What is happening to my tomato plant and how can I fix it?

This plant was somehow already growing in this raised bed when we moved in (everything else in the bed was dead). I took it out, filled the bed with fresh soil, and repotted it when it was a lot smaller. It’s grown rapidly and was doing really well after initial shock until about a week and a half ago. Some leaves have started to yellow, some are even brown and crispy. Tomatoes are still ripening (very slowly) but some have remained green for some time. I water when the soil is dry, sometimes every day when it’s really hot. I know it looks like a mess- this is my first time growing tomatoes. I haven’t fertilized, as the guy at the garden center said the soil we bought contained enough fertilizer for the season. What can I do to help?

by ThePerfectNectarine