Hi Thornton, thanks for posting what you did above. That is such a sad and difficult thing to have happened in your life as a child, and it is understandable to have struggled. To lose your sister in that way, and with your Mum and Dad trying to cope as best they can, and then the hospital stay. I can see you would have felt abandoned.
As you know, I lost my brother suddenly when I was in my 20s, and looking back I didn't have great coping skills then. But I was an adult at least. To have tough things happen as a child, I think we develop coping strategies to help us feel safe at that time. They may not always be the best strategies then, but they work for us. However, sometimes we carry them into our adult lives too, and use them - perhaps without realising. It is good to see these patterns and understand what is happening for you. Though I understand that feels painful and you feel raw.
One thing I have seen posted here is to listen to your inner child when you feel these strong emotions. Sit on the couch with that frightened boy and hear what he is saying. Understand him, and you can then make choices to act from an 'adult' state and not be driven by fear (or inner child) state.
So, inner child may say - I need to find someone who I can fix and they will always be with me and not run away. So, that needs to be someone with some issues or damage (probably all of us to some degree ) That makes perfect sense to the part of you that is scared little boy who feels abandoned. If I can just get to a place where someone won't bail on me, I'll be okay - safe.
However, from an 'adult' place, starting a R with someone who has significant issues is more likely to lead to an unstable relational situation...
Of course, I am no expert and I'm only posting from my own experience - of therapy and of what I read and see posted here. But I hope you'll follow this avenue with your therapist. You said you felt you had got to a good place previously, but sometimes there is a little further delving to do, which isn't easy - but if you can recognise patterns which don't serve you well and make peace with the past, these are prizes indeed.
What I would like to suggest is that you have a look at Guy Winch's emotional first aid TED talk as I think there is some useful insight in there on looking after your mental and emotional health. Also, have you considered some practical, actual steps you can take to support your wellbeing and manage anxious feelings when they do come? I'm thinking journaling, meditation, yoga, gratitude journal etc. These are all things I have turned to at times when things felt overwhelming..
Hope this helps anyway my friend, and one of the great things about this forum is - it is open 24/7 and is always here if you need it.
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus
) That makes perfect sense to the part of you that is scared little boy who feels abandoned. If I can just get to a place where someone won't bail on me, I'll be okay - safe. 
