This was the top of the fifth inning . . . you had five more innings at bat to score one run to tie and two to win. Come on, a one run lead is definitely not insurmountable, so why blame your loss on one play? Sure, it's frustrating, but it sounds like you let that get to you the rest of the game. And yeah, why throw something way outside to Sheffield? Perhaps you were afraid he was actually going to get a hit with two strikes to take the lead.
Also, maybe examine your CAP's defense at catcher. When I put guys like Carlos Santana or Gary Sanchez behind the plate, it just doesn't compare to Posey's superb blocking abilities for stuff in the dirt. Some people really sleep on defense ratings on certain positions like C or 1B that don't get the ball hit to them in high frequency.
Maybe switch to pure analog instead of meter pitching for increased levels of user input. It's worth trying at least if you haven't already . . .
About spending money on the game, that probably makes you invest more time and effort than you otherwise would. I know a lot of people do it, but spending $1,500 to get Mantle, Sheffield, and Biggio doesn't automatically increase your chances of winning in this game. Plenty of free cards out there who rake (i.e. POTMs, future stars, TAs). So maybe you wouldn't feel this let down if all you did was spend $60 upfront, but no one's stopping you from buying stubs in-game. Try giving it a rest for a day or two and think it over, that might help you see the bigger picture of issues like this a bit more.
But seriously, this forum has turned into a cesspool for players complaining about in-game issues and frustrations. One might wonder if that's the primary focus of users discussing MLB the Show 20 on this forum. It's a simulation video game; no one expects it to be a perfect experience, much less to win every game. If you don't like it, no one's forcing you to keep playing. Maybe the card drops throughout the year, including collections and new pack releases have sucked you in to spending tons of cash every year. Hey, it's making a profit for SDS. There's plenty of other things to do now and in the next few months, find something you actually enjoy I guess.