I have insecurities with my gf I dont want to shes never giving me a reason to be that way my ex messed me up ?

How do I overcome them I want to be with her I want to keep her as my gf I cant lose her I messed up three times not cheating thats not me I got all upset a few times by my brain thinking she was with another guy and she wasnt please help me I want to make things better I told her I will get over them the best way I can I believe I can do it she dosent n it hurts wen she dont believe me