Well first you need to stop feeling bad and start being motivated, you're only 15 you got a lot of time ahead of you.
Start lifting as heavy as you can and eat A LOT, as for the workout, you can...
keep going, adjust your calories if you stopped losing weight, the last bit of fat is usually located at the bottom layer of your stomach, you're probably near this
Proper diet is the key, maybe a little cut would do it if you say you have chest fat(while still lifting as heavy as you can), a proper cut will tighten up your chest for sure(if you have enough...
Now you look skinny(probably)
if you gain muscle, you will look less skinny until you won't look skinny at all
and being "skinny" while having lots of muscle is what most people want, well I guess
I got injured badly when I was 14 while playing soccer(knee surgery), when I squat my knee hurts a little near the end of my sets(the injured one ofc), which is normal for me, but you shouldn't get...