Hello Ghoum
Basically this way to go back to the Moon is allowed for players who beat Hoenn Elite 4. As you're still doing history, you should be able to go to the Moon again by going on top of Sky Pillar and talking to Jackson. Be careful, he will still ask you to pay 15k per trip, so I advise you to be well prepared !
Just in case and if you're facing some difficulty to beat Deoxys, catch a Pokemon with Sturdy, teach it Toxic and use a lot of Revive while Deoxys is dying from poisoning. There's an abandoned Onix in Granite Cave, and you can find TM Toxic in Fiery Path near Lavaridge Town.
I wish you good luck !
:Sing:
Thank you so much :)