How can one forget a girl he has loved since childhood, but cannot propose to her now due to his young age and financial situation? Is it permissible to send her a message asking her to wait, or to pray that Allah unites them in Paradise if she is not destined for him in this world?

If you are able to marry the girl you love, then proceed with it and seek Allah's help with the marriage expenses. The Prophet, peace and blessings be upon him, encouraged marriage, and Allah promised richness to whoever gets married, as He stated: "If they should be poor, Allah will enrich them from His bounty." However, if you are unable to marry, then you must be patient and fast. You should not ask Allah for the girl to be your lot in Paradise, because every woman will have the husband she had in this world, and she might marry someone else.