Hi OP I'm sorry you are feeling so sad and frustrated. FWIW I think you are doing the right thing by working in another job just to have something on your CV, but that is obviously not a long-term solution.
It looks to be as if you have a mix of things going on: job, childcare and friends/social support. It takes time to get over a horrible life-changing event like you have been through. Be kind but firm to yourself. Its OK to feel sad, but you will feel better quicker if you distract yourself by having a lot going on.
As far as the job goes, others are right that it takes time to find a job so stick with it. Have a good think round what you can do - could you work for yourself and consult for people who you came into contact with previously? Could you improve your CV by taking on a part-time voluntary role e.g. school board of governors, marketing and PR for a charity etc? Could you write articles for journals in your field to keep your profile up? Are there return to work seminars in your field? If not, could you organise one? Could you coach youngsters or mentor more senior people in your previous field? These are just examples, but don't assume that you have to do exactly what you did before. Have a look at Reach reachskills.org.uk for a good selection of voluntary posts.
Also, think about what additional skills would be useful: Excel? basic accountancy? Coding? Most things are available on Youtube now. I taught myself Excel (very painfully!) and I use it every day now.
It took me 5 years to find a full-time job after being made redundant at 50 which coincided with my divorce. Oddly enough, the job I ended up in uses every one of the miscellaneous skills I acquired during those 5 years.
Childcare is the bane of the working woman, but yours will start to get easier soon. Does your school have an after school club and holiday clubs? If so, I'd enrol them as much as money will allow. Failing this, can you swap collections with other parents on a rota or just work on becoming very active in a playdate circle? If you have the funds could you hire a teenager for a couple of days a week to play with them after school?
It does also sound as if you need more of a social life. School PTA's are in many ways a pain in the bum but involvement in them does mean that you get to know lots of people and therefore get invited to more things. Try and play an active role even if it is not your scene because it does open doors to friendships which will probably last for years.
What hobbies do you like? I think you need one evening a week which is devoted to you doing what you want to do (preferably with other people). Apart from this, could you and the children join a family swimming group at the weekends at your local leisure centre? Or a walking group? Your children are old enough to do walks now. Cheap Saturday morning cinema club? Is there a Gingerbread or lone parents group in your area - if not, then start one - there will definitely be other lone parents around who want to meet people; check your local library for events too - reading groups, book days etc. (also a great place to borrow DVD's cheaply for a Friday night film night with the children and one or two of their friends - Other parents LOVE you if you take their children for a Friday night sleepover and you can then ask for babysitting in return). Check at your local council too for a programme of free or very cheap events both for families and children especially in school holidays.
I found it helped to set myself targets e.g. to go out on my own once a week, to do one playdate and one organised event with the children once a week. Go to everything you are invited to whether you want to or not for the next year.
I think you also need to consider how you can look after yourself. Eat the best you possibly can as a form of self-nurturing, maybe take a multivitamin and some biotin for your hair for the next few months, relaxation, yoga etc and maybe something quick but tough like HIIT which I find a great mood lifter - again it's all on Youtube (check out Joe Wicks who manages to make it fun) - perfect for us single parents after the children have gone to bed.
I don't want to sound like a know it all because I really don't claim to have all the answers, but I have been through having to rebuild my life as a lone parent and you definitely do get back what you put in. It's all upwards from here - honestly.