I know exactly what you mean. I am always feeling guilty for not spending enough time with ds. I am happy to be working full time though as I am not a stay at home mum. I wish I was and admire those that are but realise it's not for me and best for ds who gets quality care and attention from his childminder with no ratty mother.
Getting home at 6.45 is late. I try to pick mine up no later than 5.30 but know that isn't always possible. Definitely look into the new flexible working laws which came into play on 1 April 2003. If you have been with your company for a period of six months or more it could really work for you. It means you don't have to work part time just flexibly.
Can you do any work from home? I'm lucky I sit in an isolated office from my boss and team members so no one knows if I'm here or not and can often be found at home with my feet propped up, tracksuit on and computer on my lap. TV is naturally on but muted when anyone from work calls on my mobile. Doesn't sound like you can do that.
If you want info on the flexible working laws I downloaded some interesting stuff which gives the business ideas on how to deal with it. I can send it via email if you want. Looking at it from the businesses perspective means you can have all your arguments ready and a solid reason why.
The part I find hard is when I'm sitting on the couch at night but should actually be doing something like, dishes, ironing, mending, paying bills and what have you. You need to become ruthless at work. Cut out all the things you do that aren't your job and concentrate on what is important to you and your role. Make sure you have your lunch hour and do your running around during that time. Carry a freezer bag in your car so you can get your groceries and store them while at work. Make sure your boss is fully aware that if you work through your lunch you are doing so because you need to go earlier in the day or during the week.
It sounds like I am being a real dictator but just remember it is very easy for a business to take advantage of you and if you don't look out for yourself they won't. I work in human resources so even though we are supposed to care about you we actually don't as we have to do what the business wants and the business always wants the most cost effective answer.
Weekends are hard if you need to do something for the house or even want time out for yourself as you do feel like you are taking away something from your child that they should have. I think our times are harder in the choices we have to make. We have it easier with husbands/partners that generally do more or are expected to do more but when it comes to making a choice and believing you made the right choice is always difficult.
Are women programmed to feel guilty for everything. I think we are and I am sure I'm one that doesn't actually suffer that much guilt. I always have to reason things out though.