First of all, you cannot steal someones soul from them and pull it into your own body consciously or unconsciously. However, if these new thoughts and emotions that are not your own is new, it could be one of two things, simply a spirit link/influencement OR a link to someone physical you have or had felt possessive over such as a significant other or if they felt possessive over you whether they crossed over or not. In Native American belief, these links are "soul shards"--parts of the core energy of the individual. Luckily both of these situations can be remedied identically. Cutting such a connection and removing the energy of another, be it spirit or physical person can be done through meditation/trance or if you are not familiar with visualization through meditation you can first try a cleansing and banishment of all foreign energy from your body, mind and spirit with an appropriate bath or ritual. Then use a protection afterward that specifically prevents any foreign energy to enter your personal bubble.
A curse would not cause emotions and thoughts not of your own like you're describing.