I have noticed a guy with a 70 durability that plays the o.f after 60 games is still at 100% but my first baseman has a 77 durability and ive had to bench him twice already to keep him at 100%. So do different positions or how much a player hits affect anything?

Different positions definitely fatigue more than others.

I am surprised that a 1B fatigued quicker than an OF, as I would have thought 1B was one of the ones that didn't fatigue so easily, but I honestly haven't given it too much attention as I just get the best players I can I rest them whenever necessary.

PAs also affect fatigue more-so that innings or so it seems.

I would guess that it's your manager settings. Your outfielder gets taken out late in games due to your player rest heirarchy, but your first baseman does not. Get someone assigned to back your first baseman late in games.

Check ST stats.  If the 1B has 50 atbats in ST or whatever, that will hurt him.

i didnt even think about ST.. I played him alot cause he is a top prospect.. Thanks..