Battery, acting weird.

Hello, everybody.
My Macbook Pro is way from mid 2009, but suprisingly still working great.

My battery health recently and for a while was around 87-89%.
Yesterday, however, health dropped to 47-55% and Macbook showed "Service Battery" message. I fully charged battery and when started discharging, after hitting 90% charge, health suddenly jumped to 91% and "Service" message disappeared.

I tried charging it again and again it dropped half-way.

What should I do?
Is there a way to fix it myself?

I could very well be the battery needs to be replaced. My Macbook Pro was giving me that issue as well with the original battery (365 Cycles which is a lot for my model). I replaced the battery and all is well again, I can discharge and recharge without any message coming up.