Aww.
May I suggest being real supportive, then saving at the beginning of the last Part? The endings are decided by (mild spoilers follow!)
1 - How many times you've upset Mary-Lynn slices the pie between possible Jerk / Average / Good Boyfriend endings, all-but-two-of-which occur during the Part that starts with the swimsuit discussion.
2 - The choices in that last part determine the ending from the list of Jerk/Average/Good endings available.
So be patient and supportive, then save at the beginning of the swimsuit discussion. If you can't intuit how the hint from the gallery applies, you can still do a little brute-force work to get it.