Need help with dealing with culture and happiness

Fairly new to the game and played three rather long runs as Rome, a problem i kept coming across is that my pops were mostly unhappy, even in provinces I had for a long time. I build buildings that are meant to make them more happy and they still become slightly unhappy, lowering provincial loyalty. Espcially in newly conquered lands it feels impossible to get the local population to get above 50 percent happiness. So my question is how do I get people of different cultures to become happy with my rule and quickly, also when and what cultures should i integrate or just leave to assimilate. I do plan on playing mainly rome for a while so tips while playing as them would help too.