If this is anxiety related.... then what you're trying to do is similar to putting a pot of water on to boil... if you sit there and watch the water... it'll never seem to start to boil. However, if you walk away for a few minutes and do something else... you might end up running back about the time you hear the water bubbling over.
Stop worrying about cumming... or how 'weird' it is for you to cum... or whether or not you'll be letting her down for not cumming that night. As cheesy as it may sound, try to focus more on the intimacy of the act and less on the mechanics of it. Admire how beautiful she is, the lovely moaning and cooing she makes, and how lucky you are to have a sight as lovely as a woman completely wanton of you strewn out before you. Partake in foreplay more... pleasure her... let her pleasure you. Enjoy the closeness... the excitement... and if in the midst of all that... you happen to cum... then great. If not, then focus on making the act special for her --- get satisfaction from pleasuring the ever living hell out of her.
In time, you'll pay attention less to cumming.... and while you are distracted... you may cum more... as by then, you'll be caught off guard.
Give it a try...
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen