The pain of being ignored in a realationship?

I've started to move on with my life after my partner pulls the "silent treatmeant" for no reason.
He doesn't respond when I ask him how his day is, if he wants to meet up etc. I always respond and make time for him in my life and have done for a year.
He will ignore me to the point of me becoming very upset, and I believe he does it on purpose to "control".
He will read my messgaes while actively being online and ignore them... Then when I "give up" he will message me like nothing has happened.
The anxiety and pain this causes me is horrible and I've decided to move on with my life now.
i haven't heard from him at all over a week untill yesterday when he wanted to see me.
I've explained how his behaviour hurts me yet he still does it.
I got so mad, depressed, lonely that I've slept with someone else and I've told him about it. I missed him so much and tried to fill the "void" with someone else.
Why do partners do this when they know it hurts the person that cares for them?
+1 y
Thanks everyone for responding.
Yesterday I went "snooping" on his Instagram and noticed some flirtatious messages between him and another girl with him calling her his princess.
I screamed and screamed at him for hours. The fact that I've been feeling so lonely and missing my partner while he sends messages like that is heartbreaking for me.
He called me back and we spoke for over an hour on the phone. I was apsoolutley devastated and he is going to come over later to talk to me.
The pain of being ignored in a realationship?
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