Thank you for your question. The simple answer is, 'yes' you can undertake a postulancy year while still continuing in a relationship, but 'no', I don't advise it. To really gain the benefits from time as a postulant, it is certainly easier if you are able to put relationships on hold for a while, to be able to focus on the experience of what it might be like to be a Brother or Priest - for the rest of your life. As a postulant you 'postulate', you 'put forward the proposal' of being a Brother or Religious Order Priest - you live the life, to a certain extent, that a Brother / Priest lives, to gain the experience 'from the inside'. In this way you can be better informed, to be able to make the next decision - whether to go on to the next stage by starting a formal period of time as a Novice, or to move on from the Postulancy knowing that God is probably not calling you to this vocation, or not calling you to this particular Religious Order.
There are many complications with this answer, of course. What of the situation, for instance, of being a postulant, and falling in love half way through the year This was certainly my own experience, a wonderful experience, but at the same time, something that I eventually had to work through and make a decision about - Do I want to leave and pursue this relationship or other relationships? Or do I want to live out the question that was drawing me in a different direction, to try further to fathom this call that I was still perceiving, even above and beyond this experience of exclusive love.
And so, the decision is yours, in consultation with your vocation director, whether to continue on in the relationship outside the postulancy or within it. It might be better to move to a stage of Postulancy only if the relationship has moved to a stage when you realise that it's time to move on. Then again, you may be sufficiently moved to bring the relationship to a conclusion before undertaking the Postulancy. Depending on the level of trust you experience in the relationship, might I suggest you not only discuss the matter with a trusted mentor, and/or someone connected with the Order, but also with your girlfriend? If this is something you are really struggling with, then maybe she will have some insights that will help you in your discernment. Tread warily though. She may not be able to see the bigger picture that you are grappling with here
God bless you Ryan in your continuing search - I will pray for you. Feel free to write again.