Have hard conversations in good conditions. If there's a serious matter to discuss, bring it up when all the person's needs are met. Don't do it when they're prone to being irritable e.g. after a hard day at work, when they're hungry/tired etc.
Realise when you introduce uncertainty. Uncertainty is inevitable but can be a big trigger. Be as clear about what you are certain and uncertain about and offer empathy. "I don't mean to throw you off but I can't commit to that right now, I'll update you when my schedule clears"
Be mindful of the space you ask for. It can worry your partner so let them know why. If it takes longer than usual, send updates: "hey hope you're well, thanks for respecting the space I asked for, still clearing my head but I can't wait until we can spend time together again".
Too much reassurance seeking isn't healthy and is counterproductive to your insecurity. It just fuels an anxiety cycle and only provides short-term relief. It can also be exhausting to the person providing reassurance and make them feel like their efforts are ineffective.