Does saja go before or after? For example, “I just can’t believe it” or “I just want to get away” or “I’m just too tired”.

Bir çeviriye bakın

hanya=saja

hanya is placed before, whereas saja is after. Have a look at the example below:
Hanya satu/satu saja (Only one)
Hanya dia yang kusuka/ Dia saja yang kusuka (Only him/her that i like)

i just want to get away: saya hanya ingin pergi/saya ingin pergi saja
i’m just too tired: saya hanya terlalu lelah/saya terlalu lelah saja
i just cant believe it: saya hanya tidak percaya itu/saya tidak percaya itu saja

some sentences sound more natural using hanya and vice versa.

you can use both hanya and saja in one sentence, it is grammatically incorrect but we do that anyway.
saya hanya ingin pergi saja, saya hanya terlalu lelah saja, saya hanya tidak percaya saja.

in informal indonesian (at least in Jakarta area) hanya=cuma/cuman, saja=doang
hanya satu saja = cuma satu doang
i just cant believe it: aku cuma ga bisa percaya
i just want to get away: aku cuma mau pergi
i’m just too tired: aku cuma terlalu capek/ aku terlalu capek doang