Constantly remind yourself that who you are with or what you have, all are temporary in your life. Just like the way you feel things change, like the seasons change, like the colours of the sky changes, these dimensions in your life will also keep constantly changing. Some things or persons will stay longer, for sure, but they aren't going to stay always. "Everything has to end, no matter how beautiful it is." At the end of the day, it's just you. You are your only permanent companion. You are your only saviour. And if your brain cannot handle it, you will be your destroyer too. It's all in YOUR hands.
Constantly remind yourself that the people/things which make you feel better are just 'bonuses' in your life. They don't 'have' to be there. You are blessed enough to have them. Those who aren't, are doing well on their own. And we all know for a fact that how effortlessly our brain is trained to get used to things. We may not realise it's impact until it's too late.
Remember that you were doing fine when they weren't there once. And make up your mind to be okay when they leave. Because they will leave. And there's no escaping it. But what you can do is be okay just like you were before. Instead of being disempowered by them, try to empower yourself without them.