Hi, former self, this is your more fit future self with some thoughts for you.
1. Be patient. Fitness isn't something you can accomplish any more than laundry is something you can accomplish. You keep working at it. Some days it goes better than others. It's not about hitting a particular target as fast as possible and then resting on your laurels.
2. Sort out the medical stuff first. You are not having trouble running and swimming because you are lazy and out of shape. (Well, you are out of shape, but you're not that out of shape.) Get your arrhythmia diagnosed and treated and you'll be so much happier. It's considerably easier to run with a heart rate in the 140s-150s than to run with a heart rate in the 180s.
3. Groups are good. Let go of all those bad memories of junior high PE class. You are not the worst or the slowest. And if you were? The people leading the group are getting paid to help you make progress. "Torment students" is not written into their contracts like it was (or so I infer) for 80s PE teachers. Having a pal who expects you to show up and commiserates with you when you are sore afterward is worth gold.
4. Build a base. Slowly. (See point 1, though this is more about the physical and less about the psychological.) It takes time for your body to adapt to new things, but it will adapt. Give it enough time, and the thing that felled you in the first attempt will seem easy. Your first Pilates class will seem like a cruel joke, but a few years later you'll be listening for the instructor to say, "Now for those of you who need extra challenge..."
5. Listen to your body, but not too much. This is about being good to your body. So if your body says "No! Ouch! Bad!" -- pay attention. Injuries will interfere with the long-term goal of staying fit, so avoid the wishful thinking approach to pain. On the other hand, it's just a body and it's not in charge of higher reasoning. If it has been a body at rest, it will tend to stay at rest -- just like Newton said. When you make it go and it says "Noooooo, I was resting!" you can tell it to hush up already and stay in motion.
6. Get smart. Things have changed since junior high. Don't assume you know what you're doing. Read. Take classes. Watch. Listen to people with more experience. (And if you could just skip that whole thing where you try to run faster by lengthening your stride, it would save you a fair amount of hip pain.)
7. Build it into the budget. This is the only body you will ever have. Tend it well. It is worth investing your time and your money and your energy into staying fit. It will pay dividends in unexpected ways, influencing your anxiety level (downward), your confidence* (upward), and the frequency of your husband's appreciative comments (way way upward). A really hard workout may mean that your Saturday is less productive. It's still worth working out hard some Saturdays.
*The confidence part is not about having a rock star body or winning any races. The confidence part is about knowing that you can persist in the face of discomfort and discouragement, and about inhabiting your body with gratitude and a measure of grace.
8. Don't give too much weight to any one workout. Sometimes exercise feels great. Sometimes it feels like a steaming pile of futility. (Let's agree not to talk about today's run, okay?) Don't let a bad run get you down. Or when you finish a happy endorphin-buzz-y run in which you can see real progress, don't assume that your next run will be like that too. Just keep showing up.
9. Be brave. One day you will go to the adult swim clinic and you will say, "I was always a weak swimmer, and then somehow I forgot how to sequence stroke + breathing." The instructor will not laugh. It will pretty much suck to be working on rotary breathing again, but you will be glad you did it. Mix it up. Stretch yourself. It's probably salutary to feel like an idiot every now and again.
10. Accept yourself, strengths and liabilities. Your particular soul has been joined to your particular body, and they are a package deal. Your particular body is weirdly flexible, which is great for yoga. It is not so great for running speed. That's okay. Running can be a joyful thing even on the days when you wish you could do it faster.
11. You can aim for the middle. In every class you attend, you will notice the experts and the newbies. You will worry that you are too much like the newbies and not enough like the experts. You won't pay as much attention to the women in the middle. But for you, a person with five kids and a job, the middle is a good place to aim. You won't stay a newbie. You won't be an expert, or at least not at this writing. But something important will change: instead of being eager to leave, you'll be eager to come back.