Lots of shadow boxing, focusing not on what you are throwing, but the rythym that you are throwing it. Get your footwork involved, don't plant your feet at all, just move and throw combinations, and make sure you are moving while throwing.
It's hard to know exactly what is going on, but my guess is you are pausing before throwing, planting yourself before throwing, or possibly not throwing combinations that are fluent, but rather a set of individual techniques, one after the other.
Sort of like playing a chord by hitting one note, then the next, then the next. All the notes need to be hit in one fluid motion, or it doesn't sound right.
But shadowboxing, lots of it, make things flow together and keep moving, your whole body is involved in every strike you throw. Combine that with a lot of focus mit work, and with someone that knows how to hold mits well (it's a often overlooked skill)
But a good mitt holder is someone that keeps you moving, moves around in a semi-realistic way and has active hands. They are swinging back at you, varying target position slightly, etc.
Next piece is isolating what you get stuck up on and making a drill to overcome it. If you have a hard time responding after someone attacks you, create a drill around counter punching. Keep it simple at first, they come in and attack, defend and chase them back with a cross everytime. Then get them throwing combinations, come back with a cross (or a kick or whatever) immediately. If this is where the problem is you would end up having to adjust the way you defend as you would likely be backing up and setting your weight, which makes it hard to drive forward without reseting your weight.
If the problem is being unable to attack with a combination, often due to caution of a counter attack then take the same idea but flip it around. Go in with a combination, have your opponent throw a straight punch at you everytime, as you come in. Your goal is to get in and land without getting hit. He's throwing straight everytime, so you are forced to attack on angles, which makes it very hard to predict where you will be and hit you as you enter. As you get comfortable coming in and being off that line let him add other counter strikes.
Barring all that being a possible issue, and it is simply a "Brain won't let body go" train until exhaustion, when your brain just gives up and says "**** it, you do this, I'm taking a nap" then keep training and let your body fight solo for a while
