Yo, I've experienced this with illness that took a long time to shake off. Here's what helped for me:
- sunlight
- gentle exercise
- breathwork

I couldn't work out properly, so just went for walks to get some fresh air and sunlight, and did a bit of yoga.

Breathwork (e.g. Wim Hof) was good, but I took it easy. As I started improving, I threw in a bit of cold exposure.

Once you start improving, don't make the same mistake I did of pushing yourself too hard, too early. After a long illness you need to build back slowly and continue to prioritise rest and recovery.