Serious tips to keep my erection hard?

first off, i'm not ashamed of myself that quickly so i'm asking this one openly without anon status as those questions don't gather much answers.
this one is for the men.

so i got a girlfriend and i love her a lot and i'm very attracted to her but sometimes when we have sex my penis will take ages to get hard, or it'll be "hard" but not hard hard and men know what i mean.
or when i take too long to put a condom on it just slips back into it's flacid state.
it's tiring to me to have to keep reassuring her it's not her that's the problem.
any tips are welcome.
Serious tips to keep my erection hard?
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