We have our bad times but on the whole we're fairly happy.
Here, the phrase 'On the whole' means-

• We have our bad times but on the whole we're fairly happy.
- Here, the phrase 'On the whole' means- Something that is generally true.

• On the whole
- English Meaning: generally; something that is generally true.
- Bangla Meaning: মোটের উপর; সাধারণভাবে বলতে গেলে; সব কিছু বিবেচনায়।

• Ex. Sentence: We have our bad times but on the whole we're fairly happy.
- Bangla Meaning: আমাদের খারাপ সময় থাকে, তবে মোটের ওপর আমরা মোটামুটি সুখী।

Source:
1. Longman Dictionary.
2. Cambridge Dictionary.