twinkltoz is right - it's vary different dancing with a partner. Practising on your own is useful for 'grooving' steps - so you know the basic movements automatically, or to develop some particular technique - hovering slightly on beat 2 in slow waltz for example - but with a partner you must move absolutely together and work to maintain good body contact (in standard dances).
When you first start to dance with a new partner, you must expect some problems like overbalancing, or mis-alignment on some turns etc. It's natural until you get to know each other's movements and 'signals' very well. THat's why competition dancers have the same partner for many years - if they're lucky, that is.

Are you a leader or a follower?

If a leader, try thinking very carefully about what a partner will do to match each thing you do - perhaps even learn her part.

If a follower, try to worry less about remembering the dance and pay more attention to what your partner is doing.

Dancing with a partner is completely different than dancing alone...they can counterbalance you or totally throw you off. You can get away with a lot of stuff by yourself that you wouldn't be able to do with a partner...so you might think that you're doing it perfectly by yourself, but you might not be turning the right amount or putting your foot in the right place, etc.