Although sparring is one of the most important elements of any fight training program, it is certainly possible to significantly improve certain aspects of your skillset without sparring.
Shadow sparring can be utilised in more than one way, to develop:
Technique (inc. footwork, defence, striking etc)
Create a plan for your sparring session, allocating a specific purpose to each 3 minute round, for example (boxing):
Round 1: Warm-up, general movement, continuous, loose, varied striking. Nothing too explosive or strenuous.
Round 2: Footwork. Forward. Back. Left. Right. Circle left. Circle right. Keep hands high. Maintain head movement.
Round 3: Jab technique. Single, double and even triple jabs depending upon your ability. Punch retraction to defence position. Long jabs. Short jabs.
Round 4: Straight left/right combinations. Maintaining posture/height, hip and shoulder rotation, and guard. Don't flare elbows. Don't bounce up and down.
Round 5: Speed work. Maintain defensive integrity, posture and balance.
You get the picture.
As you get more advanced, you can begin to combine elements in each round. Always maintain good posture and pay attention to technique. Video yourself. You'll quickly realise when you're leaning too far forward, for example, or flaring your elbows or letting your guard down.
There are some great resources on Youtube. Simply search for '(martial art) bag work' or 'shadow boxing' etc.
Remember, you can practice most bag work drills without a bag. There are some significant benefits to this, including the fact that when you practice without a bag, you don't have the bag to hold you up. You can't lean into thin air, so you're forced to maintain your posture and to use your own technique and strength to pull your punches and kicks back. The importance of this reveals itself during sparring and fighting; when many of your strikes miss. Learning how to miss whilst maintaining offensive capacity, defensive integrity and mobility is vital.
This is a huge topic. I've only grazed the surface of the surface. Google away, but remember that you'll make much more progress if you approach your sessions with specific aims in mind. Don't fall into the habit of working on your best techniques at the expense of your weaknesses.
Also remember that if any of your techniques are poor, then your shadow sparring will be very effective at reinforcing poor technique. In the absence of in-person instruction, there are some excellent teachers online. Spend time absorbing their knowledge and devote time to practicing individual techniques (using a mirror and/or video footage if possible). Once you come to grasp the mechanics of techniques, you can better learn to recognise when you are executing techniques poorly.
As you tire, your technique will tend to deteriorate. Focus on maintaining good technique - including breath regulation - as you fatigue. A tired fighter with poor technique is relatively easy to overwhelm.
The fact you're having difficulty visualising is common. Most people do. Visualising is a skill like any other and requires a lot of practice to be effective. If you want to get better at it, gradually incorporate it into your program, one round at a time. I don't consider it a necessary element of shadow sparring. Others may disagree.
Shadow sparring is of course only one option. Combat-specific resistance training (when supplemented by skills training) will greatly increase your efficacy.
You can implement 'conditioning' rounds into your workout, or even incorporate conditioning exercises into the 'rest' periods between rounds. Resistance training is an enormous realm, and there is a lifetime of knowledge available online. Keep things very simple initially. Pushup, squat and chin-up/row variations are all possible with minimal equipment and utilise movements highly relevant to combat.
Track your progress, so you have an ongoing record of personal bests to surpass, but overtraining is a real danger for everyone, especially an enthusiastic beginner. Learn about physical and mental recovery and recognise the fact that periods of complete and/or partial abstinence from training are essential to long-term, minimal-injury, progress.