A wonderful post from new member "Pseudonym"! I'd hate Belinda too! LOL! Statemachine's responses are spot-on. Compare his attitude to that of your bunny boiler gym mistress. I've been off for three months, convalescing from a boxing-related brain stroke, and for what it's worth, getting back into my free weights and my isometrics this past week has been tougher than I anticipated. Instead of triple 12-rep sessions, for instance, with 30kg weights on the cable cross-trainer for upper body condition, I'm doing triple 36-rep sessions with 10kg weights. I could go at it harder but what's the point? It's not about punishment. It's about pleasure, the pleasure of, for example, being able to see my wedding tackle without needing a mirror. OK, so that's a guy's take on it but I am sure you know what I am getting at. Getting back into it after 33 years is not going to be easy for you but do not let body fascists like Belinda put you off. As Statemachine said, some of these gym instructors are young and keen to the point of aggression and not therefore the most suitable instructors or guides to help people like you along the way.
I think you should certainly find an older, calmer instructor or even just a friend who can be your mentor in order to help you establish a weekly routine you can follow by yourself, without any need for someone yelling at you and discouraging you. In most gyms or, rather, serious gyms frequented by serious people, there are members who will always help newbies or born-agains out and there are always people at your level, with whom you can work out on an informal basis. If the gym to which you went isn't like this, change your gym. If there are loads of loud, aggressive men and women on steroids, change your gym. Find one with a nice atmosphere, a good range of equipment and free weights and try that out. People who are overweight and out of shape are welcomed and encouraged in serious gyms. That's part of the whole experience, part of what makes it a part of your weekly routine that you soon find you cannot do without.
I don't know you but I can imagine where you're at from your post. So, instead of killing yourself with punishing rounds of sit-ups and other forms of torture to begin with, focus for a while on getting that body moving, that heart pumping and burning that grease off. People sneer at exercise bicycles because they are a staple prop in sit-coms etc etc but, trust me, if you start off with 15 minutes a session on the bike, building up by five or ten minutes a week, the aim being sessions of at least 40 to 45 minutes, this being the minimum time to gain real benefit from it, you will start noticing all sorts of beneficial effects after just a month or two. You can alternate with the 'ski machine', which gives you more of a body workout, and with the rowing machine, as long as your back is up to it.
Do that for a month or two and come back and tell us how you feel. You might also want to play around with some light free weights, say 5 or 10 lbs, working out your arms and shoulders and your diaphragm. It's hard to describe here but, assuming you find a decent place with serious members as opposed to steroid junkies with personality disorders, don't be shy about observing others and copying what they do. And never let anyone push you into trying to up the ante before you are physically and mentally ready for it. You're not in competition with anyone else. You're in competition with yourself. Remember that and you shan't risk breaking or tearing anything as you begin the struggle to bring your body to a better place and, along with it, your mind and your whole attitude to life.