Let my preface this by saying I always lift other women up, support my friends, mentor at work etc. But I have felt this way and it has almost got worse with online dating and social media. I'm late 20s.
Examples:
At 16, 2 girls tried to steal my boyfriend and I had to work hard at keeping them away
Aged 18, I was called jealous for the way my bf was behaving with another girl. I left him and he got into a relationship with her
Aged 22, a boyfriend and I grew apart which was partially down to another woman flirting. He is now married to that woman
Aged 26, my long term boyfriend cheated on me.
It is also worth saying I have also had a very nice relationship where no other women were involved.
I'm quite good looking, smart,a nice person - but it has taken a toll of my self esteem. I think I am worth a decent relationship but men don't seem to think so long term.
I have really liked a guy for a while and we were supposed to go on a date just before the pandemic. I haven't seen him since & I've realised a mutual friend is making a play for him and he seems to be noticing. It has really upset me. 
I just want one man who just wants me. Who isn't always keeping his irons in a few other fires. Is that really too much to ask? Is it a matter of maturity and age?