Can your MIL keep a book for you?
My nanny did that for me and while it makes you upset that you missed those moments at least you get to read about them.
She used to write basically their schedule for the day and any funny/cute things that Giorgia did. For eg.
9.am Arrived at house. G very happy to see me and flapping arms. Waved good-bye to mummy and had lovely hour playing on the mat. Getting very good at holding her spoon and making raspberries.
She would take pictures with her digital camera and stick them in the book.

etc. etc. In the end, I was so jealous of her having these experiences that we just cut back on every bit of expenditure we could and I too have cut back to part-time which of course doesn't help you! I (think?) I would live in a 2 bedroom apt if I had to rather than go back to full time work.

The only real thing I can advise you is to make the decision to work YOURS, ie. if you need the money then you need to go to work. Try (!) to positive talk to yourself, I'm glad I have a job, I'm glad I have my beautiful son, I'm glad I have my MIL look after him and don't have to have him in fulltime daycare etc. Remember this is probably more about you than Matthew, he knows who his mummy is and loves you, so it's up to you to realise this is just for a season and you are NOT missing out on him because you are working. You still see him every day, and you can enjoy and relish those times as very precious. I can tend to take the time with my girls for granted and get totally sick of them at times.
Lots of hugs - be careful not to get depressed - remember any time with your Matthew is still good/worth it. Keep your weekends and evening free to spend lots of time with him.
If it helps at all (?) you are not alone, a lot of us would rather be at home with our kids than at work but have to be for our financial situations.