What is wrong with me?

Hi I will try to keep this as brief as possible but there are a few things I really feel I need to get out there. I'm a 31 year old F who has never had a long term relationship. I've had a few short ones and a few 'almost' relationships. HOWever, looking back and analysing these in hindsight -I've come to the conclusion that I've only ever been the 'stopgap' girl. Only one person has ever declared that they love me and that was false as it turned out a couple of weeks later that I was a rebound and he went back to his ex. I took a break from dating for a couple of years to focus on myself and work out exactly what I wanted ..At the start of this year I felt really positive about dating again and believed that I am deserving of love and companionship. I wasn't expecting instant success but unfortunately, all the old patterns have repeated despite keeping an eye out for red flags and such. I have tried everything I can think of ,the second to last man I truly believed held promise but yet again, I found out that I was a placeholder for him until something better came along. He even asked me to agree to date each other exclusively while he carried on dating other women!(I found out afterwards) Until now, I believed I was attractive, engaging and had plenty to offer in a relationship but no matter what, the end result is always the same..I am not clingy, argumentative or standoffish ...I am basically just a normal individual but my self esteem is at rock bottom. I also want to add that I've tried dating outside of my usual type but even the 'nice' guys do the same thing to me. they all go straight on to form loving long term relationships with other women . I've researched and scoured the internet for solutions but nothing is very clear. I know that there must be something about me that is causing my situation but I'm clueless as all my friends have settled down (some who are in worse off positions than me) . I'm so sick of being heartbroken or getting my hopes up ,my feelings are never reciprocated . I'm good at spotting big and subtle red flags but even that makes no difference ! I genuinely feel like men are out to trick me because it has happened so many times and so consistently. I don't go for great looking or ambitious men either, so I don't feel that I'm 'dating out of my league ' at all. I'm so low and feel so unlovable that I don't know what to do anymore. I understand that in life , we all get our hearts broken or things don't always go anywhere but this is too much, it is always me that gets hurt and discarded. I don't believe I have any glaringly obvious flaws..I'm a very healthy weight, no medical issues or lifestyle challenges so to speak. I work and drive and have a good social circle and interests/hobbies. Doors anyone have any similar experiences of advice ? I'd be very grateful.