I don't have the feeling from you OP that you are ready to split up just yet. Can you definitely say that you are not 'IN' love with him anymore (if people haven't had abusive partners I would expect them to still have some sort of fond feelings for a partner they have been with for years, especially if they have had children together)?
You say he has had some mental health issues OP, has he sought any help for them (I do know that help with mental health issues can be very hard to come by), and have you or any other of his loved ones needed to, or been able to, support him in his quest for help? I ask this because I know how draining and dispiriting it can be if someone you care about has mental health issues, but 'appear' to make no effort themselves to get any help that they may need - sadly, the very fact that someone has an issue with certain/many of the labelled (and unnamed) mental health illnesses can affect whether they are willing to, or even want to seek help. If your partner won't seek help for himself (for whatever reason) then depending on it's severity - but you find it severe enough to mention, so it probably is, at least in your eyes, and you (and the children) are living with it, so only you can decide this - youay not be able to continue your relstionship for this reason alone.
However, if your partner is getting some help/treatment, or is at least trying to, you might want to leave making any final decisions until all available avenues regarding his mental health have been explored. If you have had enough though, and really cannot see any future together for the two of you, please don't feel guilty about that, or stay because you feel you have to - neither you or your partner will be happy (and therefore neither will your children) if you stay soley through a sense of duty - but if ypu do split, and he is willing to accept help then it would be good if you could try and arrange some sort of additional emotional support for him, maybe through his family or friends, or even his GP if the other suggestions are not possible.
If your partners mental health is not the issue, or a big proportion of it, and if you are not sure whether you still love him enough to want to still be his partner, is there any way that you can break the monotony of your life (I am assuming monotony as I certainly felt that life with young children was quite monotonous for a lot of the time, unless my partner and I made a concerted effort to not only have fun activities with our children as much as possible - it was usually things like picnics out, or family games of a cross between cricket and rounders, and camping holidays as money was very limited in those days)? We used to arrange date nights to look forward to, which could have been a walk in the countryside, ending up at a pub for a beer and crisps, or even an evening at home when we were lucky enough for my Mum to have the children overnight sometimes, we would make our favourite meal, have a couple of glasses of wine, and just relax and chat. Talking (not usually deep heavy stuff, just communicating with each other, and showing an interest in each other's lives) was the most important part of any of our date nights, that and some al fresco 'play' sometimes! I really think it is very important for adults to have some adult time, and individually, some 'me' time, otherwise life with young children, whilst also maybe holding down a job, running a house - even when shared jointly - can be very boring. If we have a partner, I believe that the ideal is to strive for both individual interests, hobbies etc and some joint ones. They don't have to be expensive pastimes, but I admit if you are lucky enough to have friends or family who are happy to babysit, and if they are free or cheap, and most importantly trustworthy as well, then that is a great help. If you don't have any loved ones near by, could you have some sort of arrangement with parents of other young children nearby, who would like to swap babysitting duties.
If you have had the stamina and/or inclination to read all of this OP, I hope that you could understand it all - I frequently had to return to before some brackets etc to see what I was talking about! I also hope that some of the posters' on here can be of at least some help in you making your decisions - I am pretty sure that you will get some excellent replies as there seem to be quite a few very intelligent women on MN.