In everything you do, you should be practicing defense. You never hit the heavy bag without first avoiding a punch, same when you are shadowboxing. You have to ingrain those reactions your mind, so it has to be constant.
Also, think about where you are in the ring. Being a southpaw, getting hit with the jab means you are in the wrong place. Circle to your right, getting outside his lead foot, and he cannot hit you. This also opens up the left side of his body for your right hook, and gives you a lane down the middle for your straight left hand, to either the chin or solar plexus. Again, being outside his lead foot makes it hard for him to counter you.
Be more aware of range and distance. You want to be just barely beyond his reach; then you don't need to react to punches. But if you decide to, one step and you have moved into range for your counters.
It is important to practice, at all times, a defensive move followed at once by a counter. Create the opening and exploit it, or he'll just throw a second punch and hit you.