I completely understand. Have been there.
Stress raises your cortisol and adrenalin levels, reduces your oxytocin levels. Your body is reacting physically to perceived threat. Evolution didnt expect us to have modern jobs, we are designed for fight or flight responses. Sleeping is no use if you need to stay alert for a prowling sabre-tooth tiger, so your brain stays switched on in a state of readiness.
Likewise your body doesn't care about digesting lunch if it needs to get you to safety - so your body's resources are directed away from things like appetite and digestion. Short-term you may get nausea or gut-churning, or diahorrea. Your body wants to free you up to run or overcome the threat you face. So you want to eat small and often, a light diet so your body doesn't rebel.
You feel drained and exhausted by being battle-ready when no physical response occurs - most likely you are doing a very unnatural thing of just staying right there at your desk alongside that threat, day after day. Your immune system has been de-prioritised alongside your digestive processes, so you may want to take some additional vitamin supplements for a while.
Moving about helps with stress, it can trick your body into think you are responding the "right" way.
Also by raising your oxytocin levels you can offset the impact of cortisol and adrenalin somewhat - meditation and yoga can help, so can visualisation (tricking your brain into correcting hormone levels by imagining yourself in control, calm, energised, strong, centred, etc).
Or you may distract yourself by reading a great book, chat to a friend, sing, dance, watch a funny film, exercise, have hot steamy sex or masturbate - you can rebalance things not just by switching off, but by actively making yourself happier. My top tip is that a great orgasm (pr several) may help induce sleepiness when all else fails as the oxytocin release is v helpful.
Ultimately though it is the source of the stress you need to address as we aren't designed to experience very high levels of stress for long periods of time.