I think after plenty of sparring sessions you'll become more fluent with combos. If you're getting hit, work on slipping punches. Stand up and MMA in general is very dynamic. I'll throw a certain combo when I feel it's the right time it's executed. It's what makes the best fighters in the world the best fighters, precise timing and perfect execution.
Keep working the pads, focus mitts, and sparring. It'll come together. It doesn't make sense to say to yourself before you fight/spar, "well I'm going to do a jab, cross, hook" just like you don't roll in jiu jitsu telling yourself, "I'm going to triangle this guy." You just have to flow with it.