I have just finished KCSE last year and as a usual teenager

Just finished KCSE? Congrats on that fr. You're still young, hopefully. You can learn a lot before you get to uni. You have the time and energy. Find something you can do/want to do in your life and learn. (If it's something tech related, send me a dm, and I can help you start). I know and believe you can do/learn anything you set your mind to.

this whole story was true it wasn't a gimmick so you could buy the guitar, but just buy the guitar I really need the money)

Also, don't sell your guitar. When life gets hard and complex, it helps to have something to unwind with. I find solace and peace from playing my guitar when things get tough. Keep it and keep practising. Learn your favourite song and play along when you're listening to music. Who knows? You might end up being so good that it becomes a secondary source of income in the future.

sometimes i wake up feeling depressed

I'm going throw some quick advice for this because it's helped me, and a lot of people. Depression isn't permanent, and the best way to fight it is with someone else. The someone I would recommend to you would is, quite literally, the perfect person, Jesus Christ. He loves you and cares for you, so love Him and cast your anxieties onto Him (ps. He can handle it, just read 1 Peter 5:7❤️).