Sex troubles

It’s been on my mind a lot lately but never truly knew how to put it in words . Me and my boyfriend of about 1.5 years have been having issues with our sex life . I can never seem to finish out our session because I find myself BALLING my eyes out crying . I 100% trust him. I 100% feel safe and loved but for some reason after a certain amount of time I just black out and cry . It’s so embarrassing to me because I have no clue what is causes this . He is soooo supportive and understanding however I feel like I’m letting him down when all I can say is “idk why this is happening “. Can someone PLS give me any suggestions or tips