what do i do

i feel like im always doing something wrong i have been with my girlfriend for about 3 years now we are young 21 and 19 and we have a child together and amazing baby boy...she has no trust in me at all she thinks i cheeat on her and think of other women when we are having intercourse.... i try to tell her shes the only one that i want but she thinks she not good enough please give me advice about what i should do in this situation