I feel so lonely too going thru this, only because I broke up with my man. Mostly because he doesn't seem ready to be a dad like he says. Its probably the worst I have ever been treated. What has helped me was moving back in with my family, admitting to myself I made a huge mistake, start to work on my self-esteem (which I didn't notice he took from me), and rebuild. Its taken me weeks to do, but I needed to. Maybe what would help is just let some friends go that don't matter and move on to friends who do. Maybe take your 5 year old to the park or something to let her play with kids and those kids moms strike up a conversation with and make friends that way. It might help.
That's how it was with my first, it sucked but of your friends were real they would take some time to hang out with you instead of partyin. Thats how o lost alot of ppl I chilled with. Not sayin they had to give up their lives for me but still can chill every now and then.
You are definitely not alone. My friends have kids and I have a boyfriend and I still feel really lonely. It must be the hormones lol just try and get out and do things that you enjoy. Sometimes it's the little things. Like I am obsessed with nails so I paint my nails like every other day because I don't get to go out and do the fun stuff. My boyfriend always works and we work opposite schedules. Its definitely stressful and depressing but it will all be worth it when the baby comes. Just take care of yourself first.