I don't know what to do about my EX. Help :(?

I will make this short.

My ex and I haven't been In contact for about 6 months. I've recently contacted him as I really want him back in my life.
His now seeing someone else and said that we can just be friends...
I agreed and am making a go at showing him how much I've changed.
( we broke up due to my insecurities and issues)
his told me that he is going on holiday with this girl but isn't sure if he will carry things on with her...
My basic situation is that I'm struggling SO bad at being ok with him being with another girl let alone going away with her. I love and miss him more than he could imagine ( he knows how I feel ) and I feel like I'm hurting myself by smiling and pretending.
Shall I leave him to his own devices and should he end things with this girl he can come and talk to me.. Or shall I carry on being friends with him and silently handle the pain I have to endure whilst they are together.

This has been seriously shortened as long explanations never get read. Thanks.
I don't know what to do about my EX. Help :(?
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