it's okay to let yourself cry until you can't cry anymore
when you want to cry, it's okay to let yourself cry as hard as you want
to cry until you can't cry anymore
おもいっきり泣く
to cry with all one's heartとかの直訳はありますが、不自然であまり使われていないですね。大体、思い切り泣いてもいいと言いたい時にit's okay to let yourself cry until you can't cry anymoreやit's okay to let yourself cry as hard as you wantと言いますね。「let yourself〜」は「我慢しないで〜」という意味です。
when you want to cry, it's okay to let yourself cry as hard as you want
when you want to cry, it's okay to let yourself cry until you can't cry anymore
泣きたいときはおもいっきり泣いてもいいよ
ご参考になれば幸いです。