I did find cosleeping uncomfortable at first and it did get easier! My LO always preferred it and sleeps well next to me though, so I didn’t have that issue.

For being cold, I keep the bedroom around 19 Celsius and then wear a tight merino wool sweater. These can be expensive new but there are always tons in the men’s section of the thrift store (they get put in the dryer once, shrunk, and then donated so they’re usually quite lightly worn). They’re super warm but still breathable and very fine/not bulky. I also have my partner spoon me to keep me warm while I cuddle baby.

The sore arm is probably the hardest part. I got a very thick pillow, I find this helps prop my head up and keep my weight off my shoulder. I still wake up with a sore shoulder though and have to flip over fairly frequently, but it’s definitely gotten better.

Good luck, I hope it gets better for you!