If you haven't had discomfort with the HSG or the mock transfer, you'll probably be fine. In my case, each of those preliminary procedures were quite uncomfortable, including some of my IUIs. (This may be because I have endometriosis.) I took 2 Valium before the transfer, which helped some, but I won't lie, it was fairly painful. I just held my DH's hand and thought of our sweet little embie taking root...which he did! We just celebrated his 6 month birthday.
I don't want to scare you, and I hope yours will be painless, but I want you to know that you have a good chance of a positive outcome either way. Good luck!