First off, if you are a rep counter, try watching the clock once in awhile and lift according to time. Build up every set. Start with a 30 second set, then 45 seconds and then a minute. In other words, mix it up! Count reps one day and time sets the next.
Two, work harder. People don't like to work hard because it's uncomfortable, but you should do it anyway. Push yourself. You should be breathing hard, talking should be difficult. This is more attainable when there is an end in sight, this is when timed sets are useful. However, keep in mind that this intense level of exercise should happen only about 2 days a week.
Also, try those compound exercises I was talking about. If you're squatting, add an overhead press at the top. Just push your arms overhead without weight to get started doing compound exercises and overtime you can add weight. The possibilities here are endless.
Remember: Always work to the last point of success, not the first point of failure.
If you need to take baby steps to maintain 45 minutes of an intense weight lifting workout, then take baby steps. There is no reason to start off at a higher level than you are able and walk away feeling defeated, never to return again.