1995 Chevy S10 111,000 miles V6 4.3L

Please I I am begging for your help. If God's up there I pray that he touches you and asks you to answer this question for me. My 1995 Chevy S10 was shutting off while driving I replaced the starter because my starter went out the week after I got it ran fine for a few weeks after that then it started dying while running and when I would try to crank it the battery will be dead and I would have to have a jump to continue going or to get it home. So I got a new battery brand new and I ordered a brand new alternator and put it in myself even had a mechanic make sure I put it in correctly.. after installing the new alternator and new battery I would crank the truck it would run for a good 15-20 minutes and then shut off again and when I would try to crank it the battery would be dead let me remind you this is what a brand new alternator and battery in the truck.. I'm at a loss could you please tell me what could be causing this it's not the fuel pump because it's taking the gas and running fine the battery is just dying and it's shutting off.