How do I make 6 buttons aligned and same size?

I am trying to use the design tab in Android Studio to design my XML for my homescreen, however when i try and drag a button it realigned everything on the layout, eg like in the image below:

I want to arrange it so the buttons are a uniform size and aligned in a 2x3 table like the following

a b

c d

e f

I would like the image to remain in the same place as it is currently, with the text underneath also not to move.

Current XML:

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    xmlns:android="http://ift.tt/nIICcg">

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/imageView"
        android:src="@drawable/amityapplogo"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="@string/welcomeadmin"
        android:id="@+id/textView"
        android:layout_gravity="center_horizontal"
        android:layout_below="@+id/imageView"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/mydetails"
        android:id="@+id/mydetailsbutton"
        android:layout_gravity="center_horizontal"
        android:clickable="false"
        android:layout_toStartOf="@+id/myhealthbutton"
        android:layout_below="@+id/textView"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="42dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/policydetails"
        android:id="@+id/policydetailsbutton"
        android:layout_gravity="center_horizontal"
        android:layout_alignTop="@+id/mydetailsbutton"
        android:layout_toRightOf="@+id/findaproviderbutton"
        android:layout_toEndOf="@+id/findaproviderbutton" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/myhealth"
        android:id="@+id/myhealthbutton"
        android:layout_gravity="center_horizontal"
        android:layout_toStartOf="@+id/contactusbutton"
        android:layout_centerVertical="true"
        android:layout_alignLeft="@+id/textView"
        android:layout_alignStart="@+id/textView" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/findaprovider"
        android:id="@+id/findaproviderbutton"
        android:layout_gravity="center_horizontal"
        android:layout_toStartOf="@+id/myhealthbutton"
        android:layout_below="@+id/visitourwebsitebutton"
        android:layout_alignLeft="@+id/imageView"
        android:layout_alignStart="@+id/imageView" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/contactus"
        android:id="@+id/contactusbutton"
        android:layout_gravity="center_horizontal"
        android:layout_toEndOf="@+id/textView"
        android:layout_below="@+id/visitourwebsitebutton"
        android:layout_alignRight="@+id/imageView"
        android:layout_alignEnd="@+id/imageView"
        android:layout_marginTop="40dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/visitourwebsite"
        android:id="@+id/visitourwebsitebutton"
        android:layout_gravity="center_horizontal"
        android:layout_toEndOf="@+id/myhealthbutton"
        android:layout_alignBaseline="@+id/myhealthbutton"
        android:layout_alignBottom="@+id/myhealthbutton"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />

</RelativeLayout>

Cheers in advance!