Needed to get this off my chest and hopefully get some advice

This is probably going to be a long one but here goes.

I've been in a relationship with my current partner for over a year (though we've known each other for 2 years now). We got close really quickly in the beginning but things only became official after 9 months of knowing each other.

He's caring not only towards me but puts in effort with my family as well, which I really appreciate. Dotes on me a lot. My family loves him and I'm pretty close with his as well. 

There is just one catch - he is very flirty. 

I might get hate or negative comments for this but on multiple occasions that I looked at his messages, I have found him texting other girls, in flirtatious manner, and even seen him telling them he's single / unwanted, just to keep them interested in talking to him.

I have tried to bring it up to him and tell him how it affects me (it was how my 2 previous relationships ended) and that this is considered cheating (to me). It always ends up with us quarrelling, him being unhappy that I looked at his phone / messages, invading his privacy and not trusting him... He apologises and said he might have crossed the line but continues to do it anyway.

At first during our quarrels, it was a lot of crying on my part, but I have collected myself together and stopped crying as much after he told me that he hates it when I cry as it is immature to him. 

These days I choose to ask him as calmly as possible, why does he do this. He tells me that they are just friends, that he texts them because he's bored, and tries to reassure me that he loves me and they mean nothing to him.. He also says things like "I'm actually building connections so when I get my licence to be a Financial Advisor, they can be potential clients" and tells me not to overthink and that those girls will never be able to replace me. 

I try my best not to think about it, not to check his messages and invade his privacy, tried to trust him.. but I think it's driving me a little crazy. 

As an over thinker, I can't help but wonder if he's texting her when he brings his phone to the toilet, or when I'm asleep and he's not. Many different scenarios run through my mind when I'm alone.. and I constantly wonder if I'm not enough for him. I feel like a crazy person for asking him to post pictures of us on his social media so maybe when those girls see that he's attached they would leave him alone. 

These days, as he is preparing for another body building competition and I'm unemployed, I have been helping him with his meal prep every day and it's draining me out, and I find myself getting drained out of energy (both physically and mentally) and often wonder if all of this is worth it.

Does the problem lie with me and my insecurities? Or can anyone out there explain why my partner is being like this? I really don't know what to do or think anymore.