It can be rather depressing when you see your body can no longer do what it used to do well.
Well, of course your body can't do right now what it used to do well. You've had major surgery and your leg is wounded. The ends of two large bones have been cut and shaped, one of those bones being the largest in your body. Then you've had two large metal things pushed hard into those bones. To do that, your muscles, tendons and many of your soft tissues had to be pulled to the sides and your kneecap was moved aside and turned over for a while. All that has caused bruising to your soft tissues.
it's going to take a while for everything to calm down and heal.
But remember that this is only temporary. It isn't going to be for the rest of your life. Where you are now is not where you're going to end up.
You have to accept being able to do less for a while, as payment for eventually having a knee that works well and no longer has that awful bone-on-bone pain every single day.
Now, this surgery caused a lot of trauma and you won't be able to bounce back from it in just a few weeks. In fact, it takes a full year before all your tissues are completely healed. You'll be able to do most things - even ride your bike on the road - long before a year is up, but your knee is going to give you painful reminders if you try to do too much for it.
This recovery takes as much patience as you can muster, but it will be worth it in the end.
The past few days I have been having trouble straightening my leg. Is this normal? Is this a product of the swelling as well? Almost feels if i have torn a tendon or pulled the muscle behind the knee.
Yes, this is normal and it's also caused by swelling. If you've been going hard at exercises, it is possible that you've caused a little damage to the muscles and tendons at the back of your leg. They don't need to be forced - just stretched slowly and gently.
Slow down. Don't expect fast results.
Some people find it takes a long time before they can extend their knee fully.