How To Overcome Insecurities In Long Term Relationship

I have been in a relationship for six heads almost with my boyfriend who is deep down my best friend and here we are six years later still with trust issues heartache insecurities with one another my self esteem has been knocked down so low I am just trying to love and support him but it seems as though he refuses to let me in I love him so much but he makes it so difficult to love please help me out with some encouraging advice