Well, you are unhappy and you need to do something about that. Is there any possibility you are depressed - you mightn't realise - it could be worth talking to your GP about this.
The fact that you feel a deep unhappiness that is not rational, moves focus from one topic to another, affects everything and you feel is very real, even the fact that you struggle to explain it to us easily and seem to have difficulty recognising the difference between your feelings and external reality, suggests this could be the case.
Or a lot of this could be habit, an effect of spending a lot of time on your own and in the home, getting obsessed about things that wouldn't normally bother you.
You sound like someone who needs to work to feel fulfilled, busy and valued. But, it is interesting that you say you didn't find work fulfilling before. So why didn't you train for something more fulfilling? Why not think about doing that now, or in a year or two? Recognise that your business is really convenient at a time when you need flexibility but not your longer term plan.
You come across as quite passive. Things aren't as you'd like but you don't think 'what can I do to change this?' In fact you sound like someone who has a long habit of seeing themselves as a victim of circumstance, always done to, passively accepting and moaning, not doing, not responsible for themselves. A classic glass half empty person. Have you always been like that?
You also sound as though you have difficulty recognising the difference between feelings and ways it is reasonable and appropriate to act on those feelings. It's not what we feel that counts it's what we choose to do about it.
Most people would recognise that while your feelings about the gym are real, the idea you could ask your DP not to go, or continue to mind, would be unreasonable. You seem to struggle with the distinction.
More simply, get our in the evening occasionally. See friends, see a film, go to your gym. It's brilliant that your DP does bedtime and is home every evening. (Some people's work away for example). Mine was home too and I found going out with friends once or twice a month, having adult conversation and keeping that link with my previous life made a big difference to my sense of myself and my happiness.