I am 19 years old and I have never had this issue. I have been with my girl for nine months and have had the best sex I've ever had with her. But the last 3 out of for times we have had sex, I have not managed to keep my erection. I have heard that there could be many reasons. I have heard that using things like steroids can affect you, like when you cycle off them, and I am currently cycling of off Trenabol, which is a supplement that triggers your body to create more testosterone and can create an unbalance in your system. Could the sudden change in testosterone be hurting me? I also recently found out things about my girlfriend that made me a bit unhappy, such as lying about how many partners she has had. Could I be psychologically messing myself up?

If anyone has a good answer please do tell.
And I have only used the trenabol once, meaning this time has the only time I've done it and before and during it I have never had any issues.

Could you be psychologically messing yourself up?

In a word, yes. Both physiologically and psychologically.

Sounds like psychological problems, try talking to your girlfriend.
Don't use any steroids or supplements ever, unless prescribed.