Chapter 15 is pretty straightforward. The parts where your choices really matter are during the Jennifer hypnosis scene and the ending.
Jennifer Hypnosis:

If you chose to submit to Alessia previously, you will get the hypnosis scene with Gwen. If you chose to resist, you will get the Jennifer scene. Here is the most direct route to get through this scene:

Put your clothes on

Put your arms down or I want to see you

Yes or No

Let the rest of the scene play out.

The Ending(s)

There are three points that you can achieve an ending. The first is a mini-ending and wraps up the Alessia storyline. When you visit her in the forest, choose accept her offer (submit to her) or decline her offer (force her to submit to you). The choice is yours and has no bearing on the other endings.

The final ending comes down to several factors.

Several chapters ago, you had the opportunity to tell Lucy that you wanted a 'normal' relationship with her. If you did that:

And you had sex with Addison before you left her house:

You will get the Liz and Addison ending.

If you did not have sex with Addison:

You will get the Jennifer ending

If you did NOT tell her you want a 'normal' relationship AND Lucy saw you have sex with Gwen in Chapter 12:

You can choose between Mom, Gwen, Both, or Neither (Jennifer)

If she did NOT see you have sex with Gwen:

You can choose between Mom, Gwen, or neither (Jennifer)

Secret ending:

In order to achieve the secret ending, you have to play through the entire game while remaining a virgin. You can have oral sex and anal sex, but any vaginal intercourse and you won't be able to achieve what I have come to refer to as the 'Psycho Ending' :)

This was originally planned to be the 'true' ending to the game, but I decided to go in a different direction. I liked it so much, however, that I decided to slightly rework it and leave it in as a hidden ending.

Thank you for playing!!