mapDataPayload := make(map[string][]*dataPayload)

mapDataPayload[rcvData.Topic] = append(
    mapDataPayload[rcvData.Topic],
    &dataPayload{Message: rcvData.Message},
)

mapDataPayload[rcvData.Topic][0]
mapDataPayload[rcvData.Topic][1]

make(map[string][]*dataPayload, 100)

{
  "test1": {
    "0": {
      "Message": "Heasdllo"
    },
    "1": {
      "Message": "Hel132lo"
    },
    "2": {
      "Message": "Hedsallo"
    }
  },
  "testanother": {
    "0": {
      "Message": "adsad"
    },
    "1": {
      "Message": "Helwqe2lo"
    },
    "2": {
      "Message": "Hel21321lo"
    },
    "3": {
      "Message": "Hel21321lo"
    }
  }
}