Great question and I imagine many people have been in similar situations. I can offer a few lessons I have learned over the years.
1. When I found myself in a situation where I didn't connect with the mission or purpose of company/project, I would eventually hit a wall similar to the experience you described above. If you can't find any way to connect with the cause, I would at a minimum be open to making a job change.
2. There have been many occasions where I felt like I couldn't influence something to make the outcome more in-line with my ideal outcome. Two changes in my behaviors greatly reduced situations like this for me. First, Be open and honest with your manager. Explain to he or she that you feel more connected to a task/role when your input is valued. Keep in mind someone valuing your input doesn't always mean it will be implemented or acted on at any greater frequency. Second, I used to focus on what I can do to produce positive results without much consideration for others needs. One of my mentors suggested I spend an equal amount of time learning what is important to my managers/leaders and peers. Doing so enabled me to think more deeply about the outcomes I was striving for and how those would resonate with the folks calling the shots. I started to proactively address concerns I new would be present and any proposals?ideas I shared were positioned in a way that would show consideration was given to all the main issues that were important to them. Over time, I earned a greater degree of trust and my leadership team started to take my advice/input more seriously. One final thought and related to what I previously shared. Your situation above makes it seem like the spec for a feature was handed to you after it was already determined it would be implemented the way it was provided to you. In my experience, that has almost always been to late to influence change I viewed to be favorable. Circling back to my first point above, being more open about the need to contribute and share ideas will hopefully open the door for you to get involved before all the details have been finalized.
I could share several other examples from my work experience. On a more personal note, I noticed you are doing this for your BF. In the short term this might make him or both of you happy. I was in a somewhat similar situation before and the resulting unhappiness from doing work for an organization that had goals/mission I didn't connect with started to negatively impact my marriage.
I could share several other examples from my work experience. On a more personal note, I noticed you are doing this for your BF. In the short term this might make him or both of you happy. I was in a somewhat similar situation before and the resulting unhappiness from doing work for an organization that had goals/mission I didn't connect with started to negatively impact my marriage.