Your finances are fine (you might consider alternatives to so much personal training), your concerns about work are an issue. No need to tell us more than you want, but you might like to think about a few things.
For instance, is overtime mandatory or expected? If not, stop doing it now. If yes, try to limit it or make it more predictable. If you are struggling with stress at work you need to protect your time to decompress and relax.
Secondly, is there anyone at work you can look to as a mentor? That might be your boss, it might be a colleague who has been there longer than you. Someone you can talk to about what is causing you stress and who might have solutions.
Thirdly, if you have only been at your job for 3 months don't be afraid to ask questions. You are still new, still learning, not just about the technical content of your job but also about your place in the organisation, about the personalities who work there and the relationships between them, about the geography of the building you work in, about the facilities you have to work in/with, about the procedures for getting things done. Don't be afraid to ask about things you don't understand, or just to confirm things you think you do know. People you work with would rather have you knowledgeable and unstressed because that makes their working life better too, so you shouldn't get any negatives from asking.
Fourthly, work is a marathon not a sprint. It is difficult to judge how you are doing when you are only a few months into a new career, but you've got your foot on the ladder, and now slow and steady is the way to climb it.
