Lotherwise of good advice on here, check out Chicos links.
It does get easier you are early days. Important to take pain medication and do your exercise regime gently at first building up. You just have to work through that. It's a no brainer major op major rehab!! However you will get to the stage where the exercises help the pain and stiffness. and I found doing a set 2 hours before bedtime helped. Ice and elevate often. Sleepless nights part and parcel it's hell but work your way through.gets easier week 8 for me.
I am now nearly 14 weeks post tkr and am thankful for a stable joint you will get there.. Discuss your concerns regarding doing damage with your physio and make sure you are happy with the exercises. They should ensure you are doing them correctly and tell you how far to push. Remember you will only have a few physio sessions and you have to keep it up and more after. There will be ups and downs.
Don't worry about your future mobility concentrate on the now. Osteoporosis is common and they've diagnosed itv and there are good medications for it. It is good you have had your knee done as weight bearing exercise is good for that. I know it's hard not to panic but deal with now and invest in the future has always been my mantra (RA since 23 years ups and downs now 56 still mobile )
All the best and let us know how you get on
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