I will be glad and rejoice in your covenant love,
For you have seen my affliction,
You have known my soul in adversities,
And you have not shut me up into the hand of the enemy,
You have set my feet in a large place.
And as a result of trusting in YHWH he is filled with gladness and rejoicing at His covenant love, that love which was the cause of Him establishing the covenant (‘I am YHWH your God Who delivered you out of the land of Egypt, out of the house of bondage'), and which now continually reaches out to His people through the covenant. For he is aware that in His love YHWH has seen his affliction, knows precisely what he is going through, and rather than delivering him up to his enemies, has set his feet firmly in ‘a large place'.
In other words in his need God has not let him down, but has protected and established him because He is his covenant God, and has made full provision for him.