Ikkyo is so hard because it's so simple. There are a lot of technical video references and explanations on how to do ikkyo to a partner. I'd like to offer some thoughts on shomen uchi ikkyo with regards to what you could do with your own body (not your partner's) (I won't cover timing, taisabaki and other fundamental technical aspects you also need to consider):
1. I like to think of ikkyo as a sword cut. When lifting the sword, lead with your elbows (think of heavy elbows projecting or being stretched way down and forward; feel as if your humerus bone is being elongated down and forward toward's uke's feet). Move with knees projecting forward and down as well (think of femurs being elongated towards uke's feet). (When you feel these projections or elongations, do them gently and very small, not so hard or pronounced as to break your own posture.) Keep spine stretched all the time, head suspended up and tailbone dangling down. Eyes and focus well past your partner. Don't go straight to meet uke's arm otherwise you clash and end up force vs force. As you feel uke make contact with your arms, keep thinking under, under, under, under - but don't actually crouch, instead use the projections from your knees and elbows to go under uke and make them weak. Open the chest forward to add power and keep raising the sword.
2. As you reach the apex of the sword raise, your power should be "under" uke and uke should feel light/weak, otherwise you are going to have a really hard time moving them. If not I recommend checking if there was a loss of power along the way (posture, tension in the shoulders, etc.).
3. As you cut down, again lead with heavy elbows. Cut big, down and out. Engage your full back as you cut down. Don't pile your body weight on top of uke otherwise you'd be forcing yourself down and it'd be a passive force. Keep eyes and focus way past uke not on the arms. All through this point keep fingers open and extended as if there are rubber bands from fingers to belly. As you get over uke and uke's arm is close to your hip level then
(not before)you grab to finish your osae waza.
1-2-3 should be one move, like a sword cut. Train slow with good uke.