Weissdorn, I don't think that's a terribly good attitude to volunteering? Yes there are a few volunteer things you can do as much or as little as you want (being one of a team of online moderators for instance) but in most volunteer jobs ones who turn up only when they feel like it are about as much use as a chocolate teapot...
OP, I read your post with interest as your situation is rather similar to that of my DSis (although you are in a better position as you frankly acknowledge your issues and have tried to get help e.g by seeing counsellors).
From what you said in your OP and following posts, I wonder if you are a little like my Dsis, in that you are doing a good job of being a mum yet find it very frustrating at times, but don't feel you can delegate the task to anyone else. DSis seems to get very anxious about leaving her DC with anyone, whether family members, paid carers or even her DH -- and as far as I can see it's irrespective of the competence of the carer, whether the children enjoy being looked after by them, etc. Over the years it has made her very frustrated as she now has no real interests of her own nor time to pursue them, but it's hard to help someone who won't or can't let go and trust other people to help with DC.
As regards wanting the beautiful house and garden, I wonder is it the process or the product? If it's that you'd simply like to have them, but can't afford to employ builders, landscapers etc then it would be sensible to factor some days at nursery into the equation as if you're going to do the work yourself, that is the cost of your time ifyswim? A friend of mine did up his house while his DW paid the bills with her City job, but he had no DC costs, it saved lots of building costs and he got to pursue a creative ambition at the same time (writing a novel). On the other hand he was prepared to, and did, return to work when his DW was made redundant.
If it's the process then that's a possible way forward in that when your house and garden are done you will have a useful skill base that could lead into satisfying work in building or horticulture (I would say interior design or garden design but I think those areas are very very competitive at the moment). As an interim step, although you say you don't enjoy mother and baby groups would you consider setting up a group that would include children and parents but focus on gardening, craft, design etc? I know I often enjoy things more when I have organised them.
If it's about using those tasks as a form of therapy, again, it might be worth the investment in the childcare so you can do them properly and again it could lead to an interest such as therapeutic horticulture. There are organisations that specialise in that such as Thrive (www.thrive.org.uk).
I suppose what I'd try to do (and what I'd love my DSis to do) is to stop telling myself "I should" about everything all the time and work out what would actually lead to a more satisfying life which might include taking turns with DH to earn a living/him becoming the main carer for a while if feasible and work out steps I could take towards those.
We don't all have to be exactly the same or 'do' motherhood (or life come to that) in the same way to be a success at it.
However, I don't think many people get to have proper lie ins once they're past the teenage stage though!