You are not alone. I am 7 months out...and while sex is better for me...there are still days i am overwhelmed with images and i break down.
For me, when i am having trouble during sex, i stop. My WH is very supportive and he holds me when i cry, we talk about exactly what is going on in my head and we work thru it together.
Another thing that has helped me tremendously during sex is to find out things they didnt do together. Doing things with my WH that i know he didnt do with her helps to keep the mind movies at bay and keep me focused on us rather than them.
Everyone has a different time frame as to when it gets better. I think a lot of it depends on your WS and how supportive, remorseful and committed they are to your healing. Dont push your feelings down...talk to WS about it and work thru it together.
hugs to you