Walaikum assalam wa rahmatullah,
It is a given fact that our worship is not perfect. This is among the wisdoms of the prophetic practice of saying Astaghfirullah (‘I seek the forgiveness of Allah’) three times right after completing the prayer: one realizes one’s shortcomings in fulfilling the rights of Allah.
At the same time, Allah has informed us that, “Allah does not burden a soul with more than it can bear.” [2: 286]
Among the things scholars mention about presence of heart in prayer are:
The way of attaining presence of heart during prayer is to realize that having presence of heart with Allah is something sought during every moment in one’s life, and that the means to it is doing as much dhikr as one can. This is not to churn up the reward alone, but in order to be thankful of Allah, to express one’s love of Allah, to attain unto closeness with Allah, and in order to strive to fulfill the duties of slavehood to Allah Most High. Keep a small prayer bead with you and keep making dhikr, such as “La Ilaha Illa Allah” or “Allahumma Salli `ala Sayyidina Muhammadin wa Aalihi wa Sallim…”
This, coupled with avoiding the haram, outwardly and inwardly, are keys to presence of heart with Allah.
Also, don’t just “jump” into prayer. Rather, take out a moment to focus your heart on Allah before starting, and then start. Before starting, make a quick heart-felt dua that Allah grant you love, thankfulness, sincerity…
Allah has promised us in the Qur’an:
“As for those who strive in Us, We surely guide them to Our paths, and lo! Allah is with the good. [29: 69]
Muhtar Holland’s translation of Imam Ghazali’s Inner Dimensions of Islamic Worship is excellent.