Meltdowns are exhausting, overwhelming, and uncontrollable and often the person having the meltdown will experience a huge amount of shame after. It is important to be aware that it is not their fault. Keep your voice low and calm and ask them what they need from you. Some people may be unable to answer questions, but knowing you are there with support at hand will help hugely when they are able to engage with you.
Always remember to have some time yourself when you are able to, whether that is putting on some earphones and listening to some music or grabbing a bath or taking a walk... Self-care is vital to refresh and recuperate!
If you are struggling with how to cope during your partner's meltdowns, it might be worth accessing some support so you can talk things through with someone who understands autism and can help you to process any feelings you have stored up.