Generally you need better defenders. Not just only at SF where your need is greater. "Only" strong/proficient against a pure PF or C(prob Marvelous-Phenomenal IS) wont cut it. And if he is foul prone, you´re in deep sh*t defensivly. Even with a 2-3 zone he will be slaughtered.
Better defense on your guards also helps a bit, defending against the pass inside. If your ID does a good job the opponents is forced to shoot outside shots (2p and 3p) and good OD helps to stop them from scoring outside.
If they shoot 15-20 threes on your ass, it´s because your ID is working =)
On the other side, your offense also contributes indirectly to your defence.
If you have a great outside shooter in your SF, and runs Patient your team will get him the ball (if you have sufficient passing and flow) and he will drive past the hopefully not-so-good-OD SF who is actually a PF/C playing SF.
If you run a faster offense, rebounding will be more important. A 2-3 zone would help here.
You´re up against a couple of really good team and I find that your defense is good, but offense is a little bit left behind.
The FG% is to low for your PG/SG/SF (in these games anyway), not even hitting 90p/100 and as low as in the 30´s.
Don´t beat yourself down, with a little more luck you could have won a game or two...