I went back to school when my son was 2 and was not in preschool yet. I started at a local 2 yr community college for a completely new degree and will transfer to a local 4 yr university next summer. I was in my 30s and felt weird sitting w/all these "yungins" but I had to get over it pretty quickly. That was the 1st semester and I was taking 18 hrs. I took online or hybrid courses the rest of the 2 yrs and loved it.
I did have my husband to help but since he is a full time workaholic I took care of the kid all bymyself most days, and since my husband is an outdoorsman, he was usually gone 2-3 weekends a month.
I too felt like I was missing out on my kid's development but I felt as long as I made whatever time I had for him quality time, then it didn't matter if I spent 5 mins w/him or 50 mins.
I got up EARLY (3-4am) to study, studied during his naptimes (at that time he took 2 a day), and after he went to bed at 7pm and studied until after midnight.
I did make sure I got some sleep and ate healthy and worked out, even for 30 mins. We ate off paper goods, did alot of take out or frozen meals, and my laundry room turned into our dressing room b/c I couldn't take the precious time from studying or playing w/my son to put away.
I tried not to study while my son was awake. To combat the temptation of always studying, I stuck to a strict schedule of playtime, reading, arts & crafts, outdoors, playgroups, library time, etc., never in front of the TV. So on the days I REALLY needed to study while he was awake, I let him watch TV or videos.
Don't feel guilty. Just let your son know how much you love him but right now your job is to finish school and his job is to help you by sometimes playing by himself for a long time or doing some things on his own.
I'm so sorry for the loss of your husband. I am wondering...did he serve in the military too? You don't have to answer if you don't want to. But I admire you and hold you in highest esteem - for serving, for being a single mom, for working on your education.