My insecurities make me unable to flirt or get to know someone I like, what should I do?

This is a personal issue but I don't know how to manage it. I'm a pretty extrovert and social person with most of the people around, except with the people that i feel attracted to. The other day I was at a party and a girl come to me and we made out. It was the first time I did something like that (I'm 21) and I spent a great time but the rest of the days I was uncapable to talk to her or ask her to hang out again, even tho she gave me the security that she likes me back.
This make me realize that since my last break up (4 years ago) I have a lot of insecurities that make me be more shy and unable to talk or take the initiative with people I'm attracted to. Before the relationship I mentioned, I wasn't the most flirty people of the world, but i was capable of at least talk to the people that i like (even tho i wasn't able to make the first move, but i was bravier).
So I'm really frustrated with this problem because I don't know how to fix it, because I want to be able at least to talk to the people without overthinking, like i did before. I'm not looking for a relationship per se, i just want to be able to flirt like i used to.
Has anyone know any tips?

Ps: i don't know what causes my insecurities because I really like who I am and I am not scared of rejection either.
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Ps1: I'm bi, but I'm for sure more attracted to women (like 95% girls 5% boys) so this problem it's worse with girls
My insecurities make me unable to flirt or get to know someone I like, what should I do?
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