2009 Mac has bluetooth connection problem
I have a late 2009 Mac running High Sierra 10.13.6 that has difficulty connecting to bluetooth devices after reboot. I'm also wondering how big a problem it is that this computer can't update to Mojave.
Your computer will never run Mojave as it is too old, it does not support Metal on the GPU and it cannot be upgraded.
For your Bluetooth problems unpair all Blue Tooth devices, shut down Blue Tooth, then restart in Safe Mode per Use safe mode to isolate issues with your Mac - Apple Support and restart normally. Then turn Blue Tooth back on and re-pair the devices you want to use.