Firstly, practise being at the end of your range, not so you are over reaching, but so you are fully extending your arms. One of the hardest things that beginners pick up is when they can actually hit someone, they get way to close and as a result can not get enough power or get tagged which is why they think their defence is so bad.

Secondly do not get into the habit of "hopping" after every combo, it gives a sign to your opponent that you are done with that combo. If you can stay light on your feet you can pause slightly which puts off the rhythm of your opponents defence. It also allows you to counter a lot quicker and saves you energy.

Nice work on keeping your hands high though and remember to punch at your own head height, a good idea is to stand next to the bag in your boxing stance then wrap a line of tape around the bag where your chin/mouth is so you have something to aim at.

Practise your footwork, scrap has a good post somewhere which has a string attached to you and the bag so you stay just out of range of the bag as it swings. But not looking too bad, I have seen a lot worst come in the gym so keep it up.