Not sure if you came here from Yahoo! Answers, where I have been attempting to publicise this discussion board, but variants of this question - with or without the reference to phimosis, or circumcision, ... or whatever - are pretty common.
And I am
most happy to give you the answer - as is Jim - which is that it has absolutely
nothing to do with phimosis. And whilst we regard circumcision as a terrible injury, I would have to answer the same - it would have (almost) nothing to do with it if you were circumcised - or intact. From here on, except for the last paragraph, the answer is generic:
OK, you are 20. You might as well be 13, or 15, 18 or indeed, 32. It clearly
is a factor that you have as yet, not had intercourse.
Mind you, the answer does not even change much if you previously
had!
So we know, do we not, that "young fellows" - teenagers and indeed, 20-year-olds get erections "at the drop of a hat", sometimes to their embarrassment and despite desiring
not to? Well, you may
presume from this that anything you nominate - even embarrassment - will almost certainly
cause an erection and almost nothing will
prevent one.
No.
This "hair trigger" reaction does not necessarily work that way. Yes, there is an over-excitability in the ("autonomic") nervous system, but it is not all one-sided - it works
both ways. In fact, it is
almost as common to work the other way, and that is what you are describing.
Anxiety, in contrast to
excitement, quashes the erection reflex. As well it should - while rapid erection and subsequent ejaculation might be considered "advantageous" for a hypothetical primitive surreptitiously spreading his seed in the neighbour's hut, if the neighbour is advancing with a spear,
losing the erection and running away with your "tail between your legs" (look up what
"penis" means in Latin 
) is immensely more sensible.
You didn't think you were nervous? Well, you certainly
should be for a number of reasons. Firstly, if you and your lady-friend are "really disappointed" if you can't seem to "get it together" for intercourse, then I
actually (at least from my position of immense hindsight

) feel you are placing a little too much emphasis on intercourse as such. In fact, I wonder what you really
mean by "everything except sex" because I don't really know whether what
you mean by "sex" equates to how
I understand it. For one thing, less than half of women actually orgasm from intercourse alone.
Now, you didn't mention the condom. Does that mean 1} It is your intention to have a child with this young lady or 2} Of
course you are using a condom, but omitted to mention it. Otherwise you clearly
should be particularly anxious as to whether the contraception you
presume she is using is reliable. According to the lecture I attended in the last month (yes, the meal was part of the motivation for attending, but the lecturer also credits me - in fact thanked me - for asking the good questions

), as many
births (that is, pregnancies that were
not "terminated") are actually "unplanned" as are "intended", so you really
should take that matter seriously. (And the "gold standard" in
reliable contraception is the Implanon® implant ... or the Mirena® IUCD.)
So, perhaps your
body is taking this matter more seriously than you think you are!
Or you may be concerned about Sexually transmitted diseases. You
should be because your "new partner" may
not herself be "new" to "sex", even if
you are. And that is the criterion - if she has had "sex" before, you
cannot know with any certainty whether she has some inobvious STD. (It is
very foolish to fantasise that medical tests can tell you someone does
not have a STD - they are designed to demonstrate when someone
does, not the opposite.)
Or are you somehow "put off" when your partner fails to resemble - in appearance or behaviour - a "porn" actress?
These are the main things that may be "in your head", so there is
no shortage of possibilities for your apparently puzzling behaviour in the erection department. I would suggest you address each of these - do you want any further hints as to how? Well, basically,
if you have personally resolved the rationale of actually having intercourse and taken what you
genuinely believe to be all necessary precautions, then it remains to focus
not on intercourse, but on simply enjoying your time together and allowing things to happen "in their own time". We (the "old-timers") know that "in the long run", this is just as, if not more satisfying.
And the final paragraph: Yes,
read back through this forum here,
deeply, to learn and understand what you need to do about whatever degree of "phimosis" you have. And for "extra points",
share that process with your partner.