This isn't up to us. It is God who sealed those having believed IN HIM with the Holy Spirit. Eph 1:13
And anyone having believed and been sealed IN HIM has the Holy Spirit as a deposit which GUARANTEES our inheritance as God's possession. Eph 1:14
Therefore, no one can either remove themselves from union with Christ or be removed from union with Christ.
The verse about being severed or alienated from Christ in Gal refers to fellowship. Fellowship is severed when the believer grieves (Eph 5:18) or quenches (1 Thess 5:19) the Holy Spirit.
The believer has nothing to do with being sealed. It's done TO them. We don't participate. We only receive the Spirit.
The Holy Spirit is placed in the believer as a GUARANTEE of their inheritance as God's possession. Eph 1:14
Therefore, the believer cannot e removed from Christ.
This agricultural metaphor has nothing to do with being removed from being in union with Christ.
In fact, when Jesus used this metaphor, the indwelling of the Holy Spirit hadn't even occurred yet. Jesus said He was coming after Jesus left earth.
If we lose faith and never gain it back like many have who once had it this grieves the Holy Spirit, Ephesians 4:30, and quenches the Holy Spirit in us, 1 Thessalonians 5:19 because it is the power of God that keeps us through faith. We can allow the seal of the Holy Spirit put on us to be broken if we lose faith and turn back to this evil world.
2 Peter 1:5 And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge; 6 And to knowledge temperance; and to temperance patience; and to patience godliness; 7 And to godliness brotherly kindness; and to brotherly kindness charity. 8 For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ. 9 But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins. 10 Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:
Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, 27 But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. 28 He that despised Moses' law died without mercy under two or three witnesses: 29 Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?