Trinity Sunday fittingly comes one week after the Day of Pentecost, which marked the arrival of the Holy Spirit, who, in the words of the Nicene Creed, "proceedeth from the Father and the Son, who with the Father and the Son together is worshipped and glorified..."
It is actually the Athanasian Creed, not the Nicene Creed or the Apostles' Creed, which most aggressively affirms the nature of the Trinity. We never say the Athanasian Creed in church, but yet it can be found in the 1979 Prayer Book on page 864 in the historical documents section. Here is the portion pertaining to the nature of the Trinity:
And the Catholic Faith is this: That we worship one God in Trinity, and Trinity in Unity, neither confounding the Persons, nor dividing the Substance. For there is one Person of the Father, another of the Son, and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost, is all one, the Glory equal, the Majesty co-eternal. Such as the Father is, such is the Son, and such is the Holy Ghost.
The Father uncreate, the Son uncreate, and the Holy Ghost uncreate.
The Father incomprehensible, the Son incomprehensible, and the Holy Ghost incomprehensible.
The Father eternal, the Son eternal, and the Holy Ghost eternal.
And yet they are not three eternals, but one eternal. As also there are not three incomprehensibles, nor three uncreated, but one uncreated, and one incomprehensible. So likewise the Father is Almighty, the Son Almighty, and the Holy Ghost Almighty. And yet they are not three Almighties, but one Almighty. So the Father is God, the Son is God, and the Holy Ghost is God. And yet they are not three Gods, but one God.
So likewise the Father is Lord, the Son Lord, and the Holy Ghost Lord. And yet not three Lords, but one Lord. For like as we are compelled by the Christian verity to acknowledge every Person by himself to be both God and Lord, so are we forbidden by the Catholic Religion, to say, There be three Gods, or three Lords. The Father is made of none, neither created, nor begotten. The Son is of the Father alone, not made, nor created, but begotten. The Holy Ghost is of the Father and of the Son, neither made, nor created, nor begotten, but proceeding.
So there is one Father, not three Fathers; one Son, not three Sons; one Holy Ghost, not three Holy Ghosts. And in this Trinity none is afore, or after other; none is greater, or less than another; but the whole three Persons are co-eternal together and co-equal. So that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshipped.
He therefore that will be saved must thus think of the Trinity.
If you struggle with this doctrine, you're not alone: it took the Church several centuries to clarify its teaching on the nature of the Trinity. These sermons may be of help also:
The Athanasian Creed (2010) by Fr Mead
Love is All You Need (2009) by Fr Mead
The Strong Name of the Trinity (2008) by Fr Mead
The Trinity: The God of Jesus (2007) by Fr Mead
The Trinity is Our Story (2005) by Fr Austin
Three Persons in One God (2003) by Fr Mead
The Holy Trinity (2002) by Fr Mead
Almighty and everlasting God, who hast given unto us thy servants grace, by the confession of a true faith, to acknowledge the glory of the eternal Trinity, and in the power of thy Divine Majesty to worship the Unity: We beseech thee that thou wouldest keep us steadfast in this faith and worship, and bring us at last to see thee in thy one and eternal glory, O Father; who with the Son and the Holy Spirit livest and reignest, one God, for ever and ever. Amen.