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Shadow boxing is great and there's plenty of videos online for that, or get a reflex ball, they're also good fun but only if you have to space to use it in. Same goes for a skipping rope. If you can't get out/self motivate for a run often, there's always burpees, they're the best bodyweight exercise for burning calories per minute of doing them but your legs will hate you.
The wrong muscles are taking over. Your hip flexors are activating more than your abdominals. Make sure you're contracting your abs as you do the movement, but it might be better to find some corrective exercises online. It's always a good idea to spend some time doing an exercise slowly and focusing on the range of movement to hit those muscles efficiently, like building a good foundation.
I'd help more but I'm a tad baked right now.