Interesting subject matter. Looking forward to hearing about it.

You could use Ajzen and Fishbein (1991) theory of planned behaviour. The two states- single v taken could be looked at along: Single- Attitude v taken Subjective norms. The single will more likely be motivated or not depending on their attitude towards study at university while the taken could be motivated or not in relation to the subjective pressures imposed by the partner.

You could construct your own scale as Ajzen and Fishbein (2010) give clear guidelines about how to construct a scale for such a purpose.

Hope you find this helpful.

Best would probably be to use the instrument from Reeve & Tseng (2011) measuring cognitive, behavioral, emotional, and agentic engagement - it's in the self-determination theory framework mentioned above.

Also of interest would be work by Ellen Skinner on classroom motivational dynamics. See her work in Journal of Ed. Psych (2008) and Educational and Psychological Measurement (2009).

Hi Isabella, there is a lot of intruments that asses self-efficacy for self-regulated learning (see Barry Zimmerman studies). Also, you should read the Handbook of Research on Student Engagement Christenson, Reschly, Wylie (2012). 

In our work (The role of teacher behavior, task value, self-efficacy, achievement goals, enjoyment and shame in predicting behavioral engagement (class participation) in class, manuscript in preparation) we describe the behavioral engagement as the participation in learning and academic tasks and includes behaviors such as effort, perseverance, concentration, attention, taking notes, asking questions and contributions to class discussions (Birch & Ladd, 1997; Finn, Pannozzo, & Voelkl, 1995; Skinner & Belmont, 1993). Moreover, as noted Liem, Lau and Nie (2008) the behavioral engagement is frequently treated as the students’ continuous effort, determination, and perseverance in learning congruent with the concept of persistence (Elliot & Dweck, 1988), effort regulation or effort management (Pintrich, Smith, Garcia, & McKeachie, 1993) when they encounter obstacles in learning. When doing this, other behavioral engagement forms are missed such as social participation in order to learn (To participate in class discussions or to ask questions to understand better). In general, the scales included by the researchers assess effort regulation (Pintrich, Smith, Garcia, & McKeachie, 1993), not class participation. In abscence of this type of scale, we construct a scale that reflects class participation. I recommend you to do the same.

Best regards

Take a look at this assessment of 21 validated instruments used to assess student engagement: http://ies.ed.gov/ncee/edlabs/regions/southeast/pdf/REL_2011098.pdf