My dear,
You are right to think that. Ethically, if you’re dating someone you cannot “drop” him or her for their close relation or friend. This is best to avoid discord and acrimony in a family or among friends.
When a man moves on in a relationship he can’t claim to move on if he just switched his feelings from one sister to another or from one friend to another. Doing so is cruel and leads to the person being abandoned to ask several questions like, “what does her sister or friend have that she lacks? This can lead to insecurity and loss of confidence on the part of the betrayed.
It is unfair that your partner tosses your love for your childhood friend who would have been the person you will turn to when you have a problem.
Men should please realise that with a plethora of women in the world to date and form relationships with, it should be a taboo to go after her sister or friend. This is a nightmare and can drive anyone crazy. This said, the reality is, bad things happen and the situation you have described is not unique to you.
My dear, it is not easy to cope with betrayal but life must go on. I advise that you keep your so-called friend and ex-boyfriend more than an arm’s length away from you. You are still young and you have a promising life ahead of you.
Dust yourself up, say good riddance to bad rubbish and keep moving on. I am confident that you will find true love that will not let you down.
Good luck.