Mid life crisis

Female Mid life crisis, I'm 45, and I believe I'm having one - it sucks!!

I've been with my husband over 20 years and we have 5 children, all still at home but older ones are all working and youngest is 11 so they're all pretty self sufficient to a point.
We both work full time with the usual stressors but over all - we're pretty standard.
However I'm struggling, I'm bored and questioning if this is it?
I thought it was my job that was the problem, I'm a manger and at times it can be very demanding, so have been looking and applying for new jobs. But then finding excuses not to take the job if I get to interview.
I have a few good friends who I catch up with occasionally but I'm usually whacked out by the time the weekend comes around or I find trying to schedule times friends are all free is sometimes impossible, so it's a very rare thing.
I thought maybe I needed something to look forward to, so we are currently building a house (this had been in the pipeline for sometime so not completely impulsive) but I'm finding all the decision making to be very monotonous and I can't be bothered with it.
And the worst bit I'm questioning my marriage and whether I am actually in love with my husband anymore??
I Hate feeling this way! He works hard, looks after me and the kids, we still have sex a couple of times a week, we still laugh together and most of the time enjoy been together. But if we do argue I'm ready to throw it all away because we do argue about the same stuff. I jump to a 'nothing is going to change and I don't want this for another 20years' mentality. I know it's not fair but is where my head is right now.
I've been to a few Doctors now, and I'm not depressed, my hormones are still ok, I'm not perimenopausal but I'm just so over everything. It's family, work and sleep with a few friend activities thrown in and it's not enough.
How do I break this funk? I can't afford a convertible and I don't want to have an affair.
I'm not a group activity person so please don't suggest cooking classes or salsa lessons but something has to change cause like everything else I'm sick of feeling like this too!