Deciding to exclusively feed formula

My baby has cleft palate so we had to do formula since birth. In the NICU they tried regular formula but it kept making him sick. therefore he was put on sensitive which is lactose free.

When my milk started coming in we combined feeding formula and breast milk.

However we noticed when i fed breast milk he would get super fussy. Sometimes threw up, or would have really bad gas pain that we had to give gripe. Thinking maybe lactose intolerant.

For the past week we have just done formula and he seems to be okay since then.

Has anyone had experience deciding to just to exclusively formula? I feel guilty for maybe deciding to give up feeding breast milk.