i have been in quite a similar situation to you, i was treated for depression for 11 years (and in that time tried every ssri on the planet!) until being diagnosed bipolar II in may this year. I have been on lithium since then, i started on 400mg and am now on 600mg. Im no doctor but am hoping i can offer some advice from my own experiences!
In response to your questions, lithium is a mood stabliser so therefore should keep your moods at a level if infact you are bipolar and experience mania/hypomania. what interested me was the fact that you mention being treated for ADHD in the past. Bipolar is often mistaken for ADHD, so this may infact be manic/hypomanic symptoms. its also important to realise that bipolar has different degrees of severity, im bipolar II which is characterized by "hypomanic" episodes. Generally speaking this means that i am not "full blown manic" and do not experience psychosis.Bipolar II's are also more likely to experience rapid cycling (frequent changes between depressed and hypomanic episodes). You dont mention what kind of symptoms you have experienced when you were treated for ADHD, but your doctor obviously has reasons to believe that you could be bipolar and could benefit from a mood stabiliser such as lithium. or perhaps he just wants to try you on this to see if it will stabilise you and benefit you more than an ssri.
You asked about how lithium makes you feel, well speaking from my own experience, i have only just become stabilised and i generally feel very calm and much less compulsive. I used to have a lot of irrational thoughts which are now subsiding. Im even starting to talk slower! There have also been no depressed symptoms as i used to get very suicidal and ive experienced nothing like that.I just feel calm and stable, which in turn has made me happier. My results so far have been good.
As for how lithium compares to ssri's. when i was being wrongly treated for just depression the ssri's quickly took me out of a depressed state but they then induced hypomanic symptoms in me, which is often the case. Generally ssri's alone are not a good treatment for someone who is bipolar, they need a mood stabiliser to control the dangerous highs.
Whether or not lithium will help you really depends on your diagnosis, depression alone is not normally treated with lithium, but if your doctor thinks there could be something else there, then trying a mood stabilser is a good option. Lithium will also not work well for everyone, but there are other mood stabilsers available. The best thing you can do is try it under the guidance of your doctor and see how you feel. Its often trial and error with a lot of meds!