i'm so sorry for your discomfort. i know you care for your mom, but you gotta accept something here- she's wrong.
she shouldn't be dating 3 at a time, she shouldn't be dating some 19 year old kid, and she most definitely shouldn't be dating a guy who claims to like her daughter!
u mentioned that u & ur mom used to be like best friends- maybe u've been friends for too long now. i think she's got some major issues. i'm sorry if i sound harsh. hey, i'd be concerned too.
i dont exactly know whats wrong with ur mum but here r a few steps i wish u'd apply-
~ try & make ur mom break up with ALL her boyfriends- clearly, none of them are very good influences.
~ u should stop considering dating at the moment since ur mom will tend to hook up with ur newest bf.
~ if she seems distracted or fidgety, try to make ur mom comfortable by letting her know that ur there for her.
~ observe ur mum- what she does in her free time, how she talks to people in the phone, how she behaves in front of strangers and, of course, how she treats you.
~ make a note of everything u observed and figure out if u find her disturbing or different or anything out of usual at all.
Don't be alarmed, its ok. the parent-children relationship is always in reverse and forward. sometimes parents take care of their children and sometimes children take care of their parents. its nothing unusual.
don't feel like ur lost or something because their r a lot of people who r in similar situations.
i go through something like this weekly. u just need to know how to handle situations.
and, yeah, whether u find anything worrisome about ur mum or not- go to some other older person [a person with wisdom and goodness!] and relate everything to her/him.
for some reason, i think ur mum is going through some sort of 'life drama' which is leading her to behave so irrationally.
i'm really praying i could help but sorry if i couldn't.
pls, dont feel lonely or helpless and dont blame yourself for anything.
good luck:D