There is a difference in meaning. 1 - "I had" is the simple past of "to have". It refers to a specific moment in the past. 2 - "I used to have" is the (pseudo) modal "to use to (do)". It compares a moment in the past with the present. It describes a situation that is no longer true. I recommend English Grammar in Use by Murphy. Unit 18. A paper edition is cheap and there is a PDF available on the net.

Both work grammatically. ‘Used to’ gives a bit more sense of ongoing or habitual action in the past. It also implies you don’t do it / have it any more. When I was child I used to eat a lot of sweets, (but now I am grown up I don’t eat so many). Good explanation here https://www.englishclub.com/grammar/verbs-m_used-to-do.htm