Welcome back. At your age you shouldn't be experiencing back and knee pain--so I assume they're bothering you because of your weight?
The good news is you're very young and can do this. You don't want to take this kind of weight into your 30'--50's because your metabolism slows with age, making weight loss harder.
So while you're young do it, and don't gain it back. It won't be easy--don't ever start off thinking like that--but also don't think of it as a diet--consider it a life style change that is for the rest of your life. Diet is just a temporary word that I eliminated out of my vocabulary long ago.
Start off with buying a new scale. Yes, I know--it's something everyone hates to get on, but hey it's time to face the piper straight on. Then make a plan. A rational plan for long term weight loss, meaning no weird "diets"--that eliminate foods that you like. Just eat a lot less of them.
As far as your exercise--if you're bored or don't like it--you're not going to stay with it. Find something else you like to do, even sports can be a lot of fun. Like tennis, racquetball, basketball, whatever. How about a class, Zumba, Jazzercise if you're a social person, you can meet new friends while you're working out. Personally I am a Jazzercise addict, because if I didn't have others to work out, I wouldn't work out. Machines bore me.
Never say I think I can.
Turn that into I know I can do this, and I am doing it. It's the way you talk to yourself that turns you into a success story. In fact if you have to write it down on a piece of paper a hundred times, do it. Get that into your brain solidly.
If you're not connected to myfitnesspal that is a free app on your smart phone or computer, that is a great easy tool to help you keep track of your calorie intake and exercise during the day,
if you're consistent about adding in what you're eating during the day. If you can afford a Fitbit or Jawbone UP to wear around all the time, it will sync with myfitnesspal to give you a more accurate picture of how many steps you're taking/--and then you'll get a real accurate picture of what you're doing.
Anyway good luck to you on your new "permanent"
you will do it lifestyle change.