CREDIT: kingdomcultureng.com
(paraphrased by me).
23 For verily I say unto you, That whosoever shall say unto this mountain (any identified obstacle in your life), Be thou removed, and be thou cast into the sea (be removed and never ever be a hindrance or obstacle to me again); and shall not doubt in his heart
(how? See the next statement. Also note that doubtful thoughts are not being spoken of here but rather, doubt that has taken root in the heart which is evidenced by words and action; usually depicted as fear and withdrawal from actions in consonance with your request), but shall believe (believing comes by hearing and hearing what God has said - Romans 10:17 - and this prevents doubt from taking root in the heart.
Doubtful thoughts might come to your head/mind, but hearing and hearing what God has said by your repeatedly saying it will ensure that those doubting thoughts never germinate in the heart, but God's word will germinate there.) that those things which he saith shall come to pass; he shall have whatsoever he saith.
24 Therefore I say unto you, What things soever ye desire, when ye pray, believe (comes by hearing and hearing what God said about the matter Rom 10:17) that ye receive them (your present possession, 1 John 5:14-15), and ye shall have them.
Romans 10:17
17 So then faith cometh by hearing, and hearing by the word of God.
1 John 5:14-15 Amp.
And this is the confidence (the assurance, the privilege of boldness) which we have in Him: [we are sure] that if we ask anything (make any request) according to His will (in agreement with His own plan), He listens to and hears us.
And if (since) we [positively] know that He listens to us in whatever we ask, we also know [with settled and absolute knowledge] that we have [granted us as our present possessions] the requests made of Him.
Selah!