ive been running to a worrying problem as of late with my current girlfriend, these days during sex if i even pause for a moment to change positions with her, 'slip out' of her, or even sometimes completely at random ill lose my erection or ill only be able to muster up a 'lazy erection'
...what's going on?

this happens alot more when i am drunk/tired/mentally distracted and i suppose no matter who you are if you're any of the above it'll happen to you but unless im drunk/tired/mentally distracted all the time these days (which im not) this shouldnt be happening.

ive had a past history of substance abuse and now im really scared this has come back to haunt me because i have read that substance abuse can lead to impotence and ED.

this doesnt happen all the time, when i first started going out with her i could go for an hour rock hard...is it because im getting tired of her? is she too loose/im too small? sometimes i feel next to no sensation when i am inside of her...is it me or her or a combination of both?

im measuring in at around 4.8-5.0 inches depending on how aroused i am

im assuming it's all just in my head at the moment but i cant help but worry sometimes.