That's great to hear! I can guarantee that almost every engineer has been in your shoes at some point! Luckily, if you find your interests spanning across multiple engineering "disciplines," that's quite alright because there are areas of application where you can be immersed in multiple types of engineering/sciences. For example, I'm a "mechanical engineer," but researched in aerospace applications, which facilitated my interests in propulsion and combustion of propellants. Right here I've covered your three "disciplines." What's important is really finding what inspires YOU - what do you find interesting enough to go out of your way to learn about? Do you want to make a change in the world? If so, what do you want to change? Whatever the field of application is, I bet there's a way to encompass two, if not all three, of your areas of interest :) If you have something in mind and want some ideas, let me know - I'd be happy to share some ideas!
As for your questions about what I do, my field of study is largely "systems engineering," involving mostly fluid dynamics, thermodynamics, and heat transfer topics. All of these fall under the mechanical and/or aerospace engineering umbrella. I look at the entire piping systems (supply vessels, components, flow controls, gauges, etc.) that route the various gases and liquids needed in the testing of high-powered rocket engines. The rocket engine test itself doesn't last but maybe 60 seconds to 5 mins or so (on average). But in this timeframe and ~6-8 hours leading up to test time, there is a beautifully orchestrated sequences of events going on throughout the test facility where valves are opening and pumps are initiated to start flows in one (or two, or three) pipe system(s), and valves close elsewhere to direct the flow in some direction, and then these may close and another system flows from it's source to another desired location, and so on. In short, there's a lot going on at the same time, all over the facility. I help moderate and analyze their operation on a daily basis, get data, document any anomalies, analyze, and make sure everything is in check before, during, and after test. As for design, we get to design and redesign these systems as necessary for the different requirements an engine may need. The fluids we use range from gases to liquids (cryogenics), and largely hydrogen, oxygen, nitrogen, helium, and water. It's never boring, especially when the reward is seeing a 290,000 pound-force engine hot fire every week!
My college experience was great and probably very similar to most engineering grads! I minored in math, joined a few clubs (i.e., ASME), got involved in undergrad research,all of which helped me narrow down my interests. Despite being one of the few, if not only, girl(s) in every engineering class, I'm happy to say I was not discriminated against. I did have a professor that warned me about possible discrimination from an older generation(s), and said us girls needed to be more on top of our game than the guys in our class as "practice" for the possible real world. He spent the entire semester calling on us (2) girls in class to have the answers for all of his questions. It was a little challenging at the time, but he had a good point and we enjoyed the challenge. Plus, we had the best grades in the end :)
If I could offer any suggestions, it'd be to pay close attention to what you find interesting and then do some homework on the engineering that goes in to it. This approach alone helped me get where I am today. You'll have a "job" for the majority of your life, so you definitely want to find something you'll not only like, but truly enjoy. I love my job because I'm passionate about it's field of application and want to see space exploration become not only more available, but pushing the limits and boundaries (literally) of where humankind can travel and discover. This drive keeps me going and I think it's extremely important for everyone to find this source of passion within. If you do, you'll enjoy every day of what you do :)
Let me know if you have any other questions and I'd be happy to help out!
Best of luck!