One possible solution is the "encoding/gob" standard package. The gob package creates an encoder/decoder that can encode any struct into an array of bytes and then decode that array back into a struct. There's a great post, here.

As others have pointed out, it's necessary to use a package like this because structs, by their nature, have unknown sizes and cannot be converted into arrays of bytes.

I've included some code and a play.

package main

import (
    "bytes"
    "encoding/gob"
    "fmt"
    "log"
)

type P struct {
    X, Y, Z int
    Name    string
}

type Q struct {
    X, Y *int32
    Name string
}

func main() {
    // Initialize the encoder and decoder.  Normally enc and dec would be
    // bound to network connections and the encoder and decoder would
    // run in different processes.
    var network bytes.Buffer        // Stand-in for a network connection
    enc := gob.NewEncoder(&network) // Will write to network.
    dec := gob.NewDecoder(&network) // Will read from network.
    // Encode (send) the value.
    err := enc.Encode(P{3, 4, 5, "Pythagoras"})
    if err != nil {
        log.Fatal("encode error:", err)
    }

    // HERE ARE YOUR BYTES!!!!
    fmt.Println(network.Bytes())

    // Decode (receive) the value.
    var q Q
    err = dec.Decode(&q)
    if err != nil {
        log.Fatal("decode error:", err)
    }
    fmt.Printf("%q: {%d,%d}\n", q.Name, *q.X, *q.Y)
}

https://golang.org/pkg/encoding/gob/ this package is probably what you want mate :)

unsafe.Pointer is, well, unsafe, and you don't actually need it here. Use encoding/binary package instead:

// Create a struct and write it.
t := T{A: 0xEEFFEEFF, B: 3.14}
buf := &bytes.Buffer{}
err := binary.Write(buf, binary.BigEndian, t)
if err != nil {
    panic(err)
}
fmt.Println(buf.Bytes())

// Read into an empty struct.
t = T{}
err = binary.Read(buf, binary.BigEndian, &t)
if err != nil {
    panic(err)
}
fmt.Printf("%x %f", t.A, t.B)

Playground

As you can see, it handles sizes and endianness quite neatly.

You have two errors in your code. Firstly, your struct fields should be exported for encoding/binary to see it.

Secondly, [16]string means an array of 16 strings, not a 16-byte string. So your struct should look like this:

type login struct {
    Seq      uint8
    ID       uint16
    Username [16]byte
    Password [16]byte
    Unknown1 [16]byte
}

Then it works: https://play.golang.org/p/Nq8fRqgkcp.

You want to use the encode package from go's library. Usually I like the JSON encoding because it's very human readable, but the package supports encoding to/from many formats including binary and gob which is a format designed just for what you are trying to do.

Example from the go documentation to encode to json:

package main

import (
    "encoding/json"
    "fmt"
    "os"
)

func main() {
    type ColorGroup struct {
        ID     int
        Name   string
        Colors []string
    }
    group := ColorGroup{
        ID:     1,
        Name:   "Reds",
        Colors: []string{"Crimson", "Red", "Ruby", "Maroon"},
    }
    b, err := json.Marshal(group)
    if err != nil {
        fmt.Println("error:", err)
    }
    os.Stdout.Write(b)
}

Example from the go documentation to decode from json:

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    var jsonBlob = []byte(`[
        {"Name": "Platypus", "Order": "Monotremata"},
        {"Name": "Quoll",    "Order": "Dasyuromorphia"}
    ]`)
    type Animal struct {
        Name  string
        Order string
    }
    var animals []Animal
    err := json.Unmarshal(jsonBlob, &animals)
    if err != nil {
        fmt.Println("error:", err)
    }
    fmt.Printf("%+v", animals)
}

According to http://golang.org/pkg/encoding/binary/#Read :

Data must be a pointer to a fixed-size value or a slice of fixed-size values.

So you can't use slice []byte in your structure. But you can use fixed size array for it.
Like this:

package main

import (
    "bytes"
    "encoding/binary"
    "fmt"
)

type T struct {
    A int16
    B int8
    C [256]byte
}

func main() {
    // Create a struct and write it.
    t := T{A: 99, B: 10}
    buf := &bytes.Buffer{}
        
    err := binary.Write(buf, binary.BigEndian, t)
    
    if err != nil {
        panic(err)
    }
    fmt.Println(buf)

    // Read into an empty struct.
    t = T{}
    err = binary.Read(buf, binary.BigEndian, &t)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%d %d", t.A, t.B)
}

Go playground