Dear Cami, I understand you perfectly.  It is difficult to change, even if you first understand that it is right to try to be more positive and you want to succeed.  I too would like to change some of my attitudes, but it is not easy.
Perhaps you could try to take one step at a time, without adding more stress to what you already have to endure.
Try to focus on the positive aspects of your future together. When you realize that you are starting to have negative thoughts, focus on something that instead is a source of joy and positive expectation.
Also, since you work hard and study, make sure you take care of your diet, eating healthy food.  Stress and sadness could also arise from a vitamin deficiency.
I wish you all the serenity!