I am currently dealing with a difficult customer who has been through bad previous experience. How can I gain his trust?

Continue communication with her/him/institution by asking feedback and suggestion on personal basis. Once you won back her/his/institution's trust then slowly talk about business.

Honestly, I am not an expert as you mentioned, however I have the experiences to deal with this type customer. 

What I have been practising is that I will fix an appointment and will go and meet the customer personally and will try to understand the situation.If the situation is worse,then I will try to feed his EGO.Whatever the customer is saying I will join hand in hand.Once all his frustrations are out then I will build a small repport with him and will make him understand that I will be there when there is any need.This helps us to win the customer back.

You have to see first, whey he /she has been get annoyed, address his/her concern first and try to resolve first as much as you can or explain him/her real situation as much as could be share with him or her. If you resolve or address his/her concern satisfactorily then you will earn him as loyal customer.

In very short "Your attitude determines your aptitude"