Sounds like you are being thrown too hard! Once your balance is taken your nage should be able to throw you at whatever speed they want. Always breath out when being thrown; practise rolls on your own and make sure you roll over your shoulder and not your neck. Make sure you are putting the appropriate foot back when rolling to prevent an awkward twist.
I generally do forward ukemis wherever possible, but learning to fall backwards I would consider imperative. With irimi-nage uke very often falls onto their back, often too fast to turn into it (in fact the throw is different really). If ukes have time to turn I would consider that there is a large part missing from the training (examine Ueshiba doing irimi-nage).