The short answer - you're screwed!
If you have a snorkel, it's a late war year. That means the allies have dedicated hunter-killer groups with or near the convoys.
These murdering wolves won't leave you. They'll let the escort take the convoy away while they just sit on top of you. They know their chances of a death blow are minimal when you're down deep, so they'll hold onto most of their depth charges and wait for you to surface when you run out of air and/or battery.
They may drop an occasional "present" just to force you to move and use up more battery.
Check with your sound man. If the DDs are still there, it's suicide to come up and try to snorkel. Radar will see the snorkel at night and they'll hear your diesels running.
Your only hope is to creep away using silent running and very slow speed, changing course often to try to stay astern of most of the DDs. Unfortunately, you don't have the battery life left for that.
Write your will on waterproof paper, surface, and hope to surrender before they blow you out of the water.