Ah, don't quit bud.
The line-change stuff is new this season - and I know what you mean about sluggish players, but you need to learn to coach it. Set new lines if needed, change on the fly if you can. Both teams are up against the same problem, but between your 2 lines you should never have all players slugged out.
Best thing to do is keep your shifts short. If you're not scoring, make sure your goalie holds the puck to get the change. You can also conserve energy by reducing the mashing of the C button for both hitting and speed boost.
When we had no line changes, it was just a deke-fest at least now the best players get tired.
As for hitting, most of your players are border-line for weight - no real heavies - and with the weight fix, you need some beef to knock guys over. Playing with your team, I'd use the C button to escape checks and the B button to stick check rather than try and knock guys over. Your skill guys in particular, need to avoid being demolished.
Play some exis to get this stuff ironed out, then show us your moves.