Complicated question. I'll try to tackle it for you in several ways. Firstly, Bipolar Disorder
usually means the person has manic episodes. These episodes involve having too much energy, staying up many nights with little sleep due to too much energy, racing thoughts, rapid speech, distractibility, increase in goal-directed or dangerous behaviors, grandiosity, and sometimes delusions and/or hallucinations
if the episode is severe. Lithium calms these manic episodes and also prevents depression
and suicidality. Many patients complain that Lithium helps stabilize their mood but as a side effect it can dampen enjoyment, pleasure and motivation to accomplish things. This is a possibility for your boyfriend. If he claims he stopped caring for you soon after he started taking lithium, it could well be that this medication has dampened some of his feelings of love and romance for you. A switch to a different mood stabilizer
or Valproic Acid
may help recover this "emotion." wanting to look at other girls could be a symptom of poorly controlled mania: he could have too much sexual energy. I hope this has helped orient you a bit better. Sorry to hear you are going through this rough time with this situation.