Why can i not stay erect please

Hey guys I really need help I'm 16 and I've been with my girlfriend for 7 months now I know we love eachother and wanna be with eachother we have had sex many times and finished but lately I've been having this repetitive problem we go to be intimate and I'm fully hard, I always pleasure her first and as soon as I want pleasure I see I'm not hard anymore she don't mind that and gets me up easily by rubbing me and doing abit of oral then as soon as I go to put it in I see its not hard anymore whilst I'm making it wet it's still up but not fully as hard as it could be as soon as I pull out she sees its not erect anymore and it really upsets her she is 18 and has had twins she had a lot of experience with other boys and she's only the second girl I have slept with she has loads of stretch marks on her body and because it took a lot for her to trust me to show me her naked she thinks its her and that I'm not attracted to her and is now wanting to starve herself and go on diets I know its not her I love her and feel 100% attracted to her I've been trying to think of things it may be we don't use condoms as she has the coil fitted and where I'm not circumcised I need to get it quite wet to feel comfortable otherwise it pulls back on my foreskin and hurts as its tight but lately we stopped using lube and I try to get it wet myself I thought that may be a reason or another that we only do missionary lately cause she likes that I know I could get up again but she too upset to even want to try another reason maybe where she a hard girl to please and when we get going she stops me from cumming on multiple occasions then I notice I'm going soft inside her I honesty don't know what to do I know its not ED as I can masturbate fine I just really need help because she hurting and I don't know how long it will be before she doesn't wanna feel this way anymore I just want to convince her I know she not the problem at all so please help!!!