Single script doesnt run, if I separate it, it runs

Hello everyone!

I have a script that when I run it in parts, it works perfectly, but if I compile it into one, it wont run, keep saying that the column that was added in the script before it doesnt exist. Here is how I am doing it:

USE myTable

IF (NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'temp_act_bk') 
           AND type in (N'U')))
           
BEGIN
	CREATE TABLE dbo.temp_act_bk (
		id INTEGER IDENTITY NOT NULL,
		title NVARCHAR(100),
		project_id INTEGER,
		active BIT,
		jpa_version BIGINT
		CONSTRAINT pk_temp_act_bk PRIMARY KEY (ID));

	CREATE TABLE dbo.temp_status_migration(
		id TINYINT NOT NULL,
		migrated BIT NOT NULL DEFAULT 0,
		CONSTRAINT pk_temp_status_migration PRIMARY KEY (ID));
		
	INSERT INTO dbo.temp_act_bk (
		title, project_id, active, jpa_version
		)
	SELECT 
		title, project_id, active, jpa_version
	FROM
		activities

	ALTER TABLE dbo.item ADD old_id INTEGER NULL
	
	-- Insert the data from backup table into the items
	INSERT INTO 
		dbo.item (
			project_id, code, shortname, type_id,
			description, quantity, quantity_unity_id,
			jpa_version, active, old_id
		)
	SELECT 
		project_id, title, title, 1,
		title, 0, 1, 
		jpa_version, active, id
	FROM
		dbo.temp_act_bk
	
	-- Add column item_id to w_items
	ALTER TABLE dbo.w_items ADD item_id INTEGER NULL
	
END

USE myTable

IF NOT EXISTS (SELECT NULL FROM temp_status_migration WHERE id = 0)
BEGIN
	UPDATE 
		dbo.w_items 
	SET 
		item_id = (
			SELECT TOP 1 id FROM item WHERE old_id = activity_id
		)	

	ALTER TABLE dbo.w_items ADD CONSTRAINT FK_w_items_item FOREIGN KEY (item_id) REFERENCES dbo.item(id)
	
	INSERT INTO dbo.temp_status_migration (id,migrated) VALUES (0,1)
END

This doesnt get along pretty well if runned in a single script, as I said, but if I run a block and then the other, it runs smoothly. I wouldn`t mind if I didnt have to compile it into a big script, but the big script is needed. 

I also tried opening a transaction and commiting in the end, same result, it doesnt work, keep saying that item_id doesnt exists