Why BMW E46 is losing power?

Extremely vague question, but here is a list of common problems to check out (also if you don't have a service engine light on it may be something simple)...

a) make sure engine air filter is not obstructed

b) spark plugs may need to be changed

c) sludge in the engine from failure to change oil when required or using improper oil

d) Air leak in your intake manifold

e) wrong fuel used

f) clogged fuel filter

g) failing fuel pump

h) clog in your fuel lines

i) clog in your fuel injectors

j) Oxygen sensors bad

k) air mass sensor is dirty, or failing

Only a tech can tell you for sure