Oh, my goodness. All your stuff in a backpack. A whole new world is waiting for you out there. I love backpacks - a lot - but not on my bike! On my bike, I can't do without my bicycle panniers.
You'll need a bicycle rack. There's a lot of variety out there, but often the simple ones are just fine. If you have disc breaks, you'll need a slightly different type.
And then you'll need the really important part - the pannier(s). I started with inexpensive ones, and eventually got a really nice commuter bag. My current one is from Arkel. It's big enough to carry my laptop (in a padded sleeve), my bulky lunch, and a few other things. I splurged on the Arkel because I've had the cheaper bags pop off my rack when I go over big bumps. I wasn't willing to subject my laptop to that. The top of the Arkel clamps to my rack so that won't ever happen. It's also pretty waterproof. It felt really expensive when I got it, but it's one of those things that I use, and use, and use, and use.
On the other side, I use pannier I got at Goodwill - a very simple one - to carry my lock, my repair tools, and cold weather gear, so I always have those things.
And then your back will be a lot cooler - and you'll feel "cool" too!