Male insecurity. How to deal?! 😖

See I was madly in love with a guy (let's call him S) for a year n he didn't want a relationship but came and left as he pleased. Took me long to realise that the possessiveness and competitiveness has nothing to do with me but was in his nature. I moved on this year and met someone and started a long distance relationship with him (let's call him M). S heard about me moving on and as usual decided to pop back in my life, this time I just blocked him off even though a part of me is still attached to him. I just really wanted to move on to something constructive and long term. My bf M has recently become quite insecure about losing me, whereas I'm more of a homebody I barely go out n socialise, yet he gets insecure. I showed him a pic from eve of a dirty conversation between a bf and gf. He actually thought I was having that conversation with someone and blew off at me saying things don't match up, I mean I don't even know how to defend my self on that. It was a post taken off this app. I don't know what to do, insecurity is inbuilt I think, I'm not insecure by nature though I have been cheated on before but I never suspect him or make such allegations and brawl over it.. totally upset me today. Ive been in a relationship before where the guy is insecure & ends up being the one cheating, irony at its best. I don't know how to deal with this insecurity, really hate having to make unnecessary explanations, where I'm really not doing anything.