I never use sliders, but... Having the difficulty set to the highest level possible is probably 95% of the issue. With the way you describe your sliders, it seems that you are intentionally causing your offensive line NOT to block whether they get the swim move or tossed to the side.
Sounds to me like you may want to roll your sliders back a bit until you get used to the difficulty and can run the ball effectively.
Also, when you run the ball... Don't SPRINT until you break the LOS. When you sprint, the defense sprints. Engaged defenders don't SPRINT - They SHOVE and do an evasion move to get unblocked.
If you wait to sprint after breaking the LOS, the defenders tend to stay blocked a split second longer.
Hope this helps.
Later