Why does my girlfriend seem to behave jealous or protective?

I think women like to feel as though they are number 1 in your life, and it is likely to be the case that this woman has past experiences of being less than number 1, so she is going to be oversensitive about it.
And the fact that you are refusing to give up your friends, reinforces her fear that she is not such a high priority for you, otherwise you would give up the friends.... that is what she is thinking.
But your thinking you should have the right to keep your friends.... and that is true..... but, possibly all you need to do is be 'willing' to give up your friends? And then once you proved your gf is number 1 priority, then she may feel secure enough for you to reconnect with those friends?

I will add one thing - personally, when i start a relationship, i say farewell to my single female friends - because there are already more than enough things to make relationships difficult without having that headache on top!

This isn’t going to work.

Find a woman who has the self confidence needed to have a relationship with a guy who has male and female friends.

I dont know, he does not sound like he is an overly self confident type himself? And in my experience, confident women dont have the time to even notice shy men since they got a hundred other more confident and better looking men lined up behind them already. He doesn't want that ****, believe me!

Get rid, she wants to control you. This is how coercive behaviour begins, it will only get worse.

Read this article, anything sound familiar?

https://www.bbc.co.uk/bbcthree/article/81a8f303-5849-45b8-85a0-e8532b5d948b

This gf is unwell to say the least. Continuing a relationship with her will make you unhappy an unwell. Breaking it off with her permanently and completely is a must. That’d include any and all social media. There is no sense continuing. She has shown inability to be reasonable or having good judgment when she won’t “allow” you to converse with other females. Even a parent wouldn’t and shouldn’t do such behaviors.

She may need professional help but that sort of level of help is beyond your expertise and shouldn’t be attempted. Y out can’t help someone with such poor self-confidence. Doing so will drag you down and both of you would suffer and be unhappy.

You’re self-respect and seeing your other friends are important to you. Reconnect with them. Any status you might get from socializing with her just to be in a romantic relationship of some sort is misdirected.

Don’t listen to anyone who says that you should give up your other female friends. They either are daft or have no relationship experience.

Any partner worth their salt should be interested in your wellbeing and that means encouraging you to have a social life that includes others - even those of the opposite gender. (Of course that would mean non-romantic activities or dating.)