You know what, OP? This word 'boring' is beginning to get to me. You describe your life, very circumscribed by young children - as many people's are - and containing few things 'hobby and calendar-wise'. So you could say, I don't have much time to do anything other than looking after the children, housework & having a bath, does everyone get this with pre-school kids? But you seem to be judging yourself.
'Boring' is a very judgemental word; I'd hate to be thought of as boring. Quite rightly, you point out that you've done a variety of things in your life, you're light-hearted and not a boring person to talk to over a cuppa. So where does the 'boring' come in, then? Purely in the matter of hobbies. You don't work out, you don't play any musical instruments, you don't go anywhere, etc. And that's because of your current responsibilities to your children. OP, I wouldn't use the word 'boring'. Quite apart from the fact that it's very easy for anyone to feel inadequate when comparing themselves to celebrities and superfit people on Instagram - and I really strongly suggest you stop doing that - it's not true, is it? Boring means you bore others - and you don't.
You don't do much, but there's a very obvious reason why, that anyone with kids would understand in a heartbeat. So I'm left to conclude that this 'boringness' is an accusation directed at yourself, by yourself, that you're inviting Mumsnet to agree with. Essentially, you've said, "I'm boring, aren't I?" And when @MrsTumbletap said OK yeah you are, you come back with a laugh and hey, I'm not BORING boring, just hobby-wise. Well, OP, that's not in any way the meaning of boring. Boring is an interpersonal thing. To be boring, someone else has to be bored. I'd go further; I'd say the word has entered your consciousness solely by means of comparing yourself to completely different people - who don't actually know you - on social media. It has convinced you that you are a less than good or adequate person and that you are boring, even though no interpersonal relationship exists whatsoever for the word to draw relevance from.
Remember you took it personally when the personal trainers unfollowed you, and you imagined them looking at your Instagram and being bored by your unfit, do-nothing self. So, you may laugh this off and that's up to you, but I'd say, your social media habits are damaging your mental health. How else does a person who has done lots in her life, has got married and is now raising children, whose only lack is a lack of time to do much else (and that happens to everyone raising pre-school kids without help) and a lack of time or energy to make plans for the future - and who, above all, is not, in real life, boring - end up on Mumsnet asking people to agree with her that she's boring?
Stop judging yourself, OP. Raise your kids, have baths, make plans. You've got nothing to be ashamed of or to feel inadequate about, you're literally doing a good - unpaid - job. And when time for yourself starts coming back, please take full advantage of it. Get fitter, play piano, whatever, and ignore all the guilt-inducing crap that's out there in the media in a million and one subtle forms, that will tell you you're being selfish.