I am 5'11" 155 lbs too. Although holding top for people like us will never be super easy, it doesn't have to be so hard either. I can easily pin bigger people with experience for minutes if I need to. You just need to work on your pinning technique.
The key is to stay as tight as possible on top. Give your opponent no space at all. Squeeze him hard and try to crush him with all 155 lbs. When he tries to flip you from side control, sprawl yourself out like a board. He can't bridge you when you are really long like that.
Also, when you feel your opponent going for a good escape from your side control, use the opportunity to switch to mount. It is very demoralizing for your opponent and quite effective.
On the bottom, my best advice to you is to stay balled up at all times. You can easily shrimp from this position, and you can use your legs and back to make space to escape. Make a habit to never allow yourself to be flattened out on the bottom.
Try to take your opponent's back a lot too. Although a good sweep will work on any size, taking the back is even easier. You don't even have to move your opponent's weight at all. You simply climb around his weight onto his back to finish. Try this from the guard a lot.