It would be hard for me to guess at whether she is using this as an excuse to break up. You have to open a communication with her like now other, especially considering that you are thinking of proposing to her. If you are still guessing at how she feels, guessing at what she wants, accepting half answers from her, then you aren't ready to propose. You have to get to know her, get to know the person she is on the inside, find out what her hopes and dreams for the future are....and be sure that these dreams match what you're looking for out of your future. For example, if she says I want 7 kids, you may not agree with that, you may say, well I want three....if she is dead set on 7 kids, you may want to think about how this will affect you further on down the road. What about where you're going to live, what career paths you will both take, how you will spend and invest money, how you will deal with holidays and family, etc. Of course these things may not be able to be set in stone right now, but they are certainly things you want to discuss and learn about the person you plan to propose to. The current question for her would be, what if I decide I don't want to go back to school, how will that affect our relationships. I have a list of over 300 hundred questions that you can talk to someone about to get to know them better, let me know if you want me to post it for you.
I want to say that I hope I didn't infer in any way that you were and idiot or that I think any less of you for you job. There are people who wish they could make the amount of money you make, you should feel lucky to have a job that pays you well. If you feel that there is no way that you can move up in your job, or make more money as time goes on, perhaps you might want to consider going back to school for that 'career', but it should absolutely be a personal choice.
Letting her know about the other date is a touchy subject. You can tell her and it could upset her, destroy her trust in you, and possibly end the relationship. On the other hand, if you feel it's something you really need to talk to her about, you can let her know that you feel this distance and want to correct it. Only you know how she may react to hearing something like this.
I'm sorry to hear that you lost your mother, I know that had to be a very painful experience for you. I can't replace her or even try to, but I am here anytime you need to talk.
I welcome your thoughts, let me know if you want to talk more