In the end you are the one that has to decide. It sounds like you once found playing the piano exciting and you have lost the love for it. You can’t really motivate yourself IMO if you don’t want to do it, with music you really have to love it. This especially is true because as a pianist it is a rough road (not going to sugar coat it), if your goal is to solo with major orchestras, it is a unicorn kind of thing,and to get there is like being a top level pro athlete in terms of how hard it is. Some questions for you:
-How long has it been since you fell out of love with the piano? Just based on experience with my son and some other musicians I know, they all seem to run into those moments when they doubt themselves,wonder if they are doing what they need to. Often it comes when you are hurting (my son is a violinist), it happens when there is tension in group, when someone is auditioning to get into let’s say an orchestra, and not getting traction. If on the other hand it is a struggle every day for a long time, it is possible you simply have found you have fallen out of love with it if this is something that has been dragging on, is not temporary.
Trying to grind through something without love for it , or as an alternate, something you have to do to get to something you want to do (Organic Chem for pre meds is something like that, few people like it who have to take it, but it is an important req so you grind through it) is not likely to work for you.
Okay, so what do you do? Obviously there are alternatives:
-Study something else at the school . Since it doesn’t have a composition program, that isn’t an option.BTW composition is as hard as anything in music, if not harder, no matter what kind of music you are talking about, making a living at it is like being a full time writer, there are few who can do that., it is just as fraught as being a musician. So if you are thinking of composition, it could be you could study something you could do as a 9 to 5 and do composition on the side.
-You didn’t say what kind of music, I assume you are studying classical piano. If you are talking classical composition, you could stay with piano performance (which won’t hurt you as a composition student later, again if classical composition is your goal), since in learning piano rep you are learning how other composers did things, learning about music. Meanwhile, if you really think you would love to do composition in the classical realm, then I would recommend focusing on courses in music theory and musicology and music history as well as piano performance. Music students often hate theory, but if you really want to study composition, it is invaluable I believe (not a composer, my son is not a composer either, but I know enough about music to believe this is true). Being able to play piano can help there as well, being able to play what you write on the piano can help you hear how the piece you see in your head works. It can give you motivation to stay with practicing the piano, playing well can help with the composition (you also could get piano gigs to help keep yourself going if you choose to go the composition route if you become a solid pianist).
-As others have said, you don’t need to formally study composition at a music school (it really all depends on what kind of composition you are talking about). Studying at a school gives you obvious advantages, so you could do piano undergrad and then study composition at a grad student for example. Besides gaining experience from composition professors and coursework, it also is a networking thing, the people you work with can be mentors and also can help you with connections.
You could also start working with a composer/composition teacher as a private student, really kind of list a master/apprentice thing. This could be true even if you stick with piano performance, or decide to study something else at school other than the piano. A friend of my son, who is a pianist, did that, he studied with a couple of well known composers.
Or you could take extension courses in composition. Michael Giacinno (who IMO is one of the most talented movie score composers out there) took night courses at Juilliard in composition and movie scoring, there are programs like that.
One thing to keep in mind is no matter what you do in music, if you want to make it a career, it is difficult, it is going to be years of struggle, of not being really independent, it is going to mean doing a lot of different things to survive (it is where finishing your piano degree can help; not to be the next Yuja Wang playing with a major orchestra, but rather as a source of gig work that can help sustain you or teaching kids). Composition if you really love it isn’t easy at all, whether classical or pop music or movie music or any kind of music, if you think it is easier, it isn’t, and be prepared for that
The key thing in the end is what is your passion when it comes to music. You have to love it to perservere, so if piano performance really isn’t your thing that unless you can find a way to see piano playing as a means to a bigger end, you won’t be able to force yourself to practice and do the other things needed in a performance program, there has to be something driving you. The sad truth is I have seen kids who were really talented, who played at a high level before college, in top prep programs, who had real talent, went to music school and then crashed…because parents and others told them they had this great talent and they ‘had’ to go into music and not waste the gift…and many of them crashed and burned because it simply wasn’t what they wanted, it was what others told them they should be doing.
I wish you luck, in the end I hope you find your passion.