I started singing again after many years and I noticed that when I sing and breath, I feel so tense in my throat at the same time. I breath from abdomen, but I feel so tense in my chest and throat area too. I have a vocal range of F3-C6, when I trained many years ago I could reach all those notes with no issue, now I feel like I'm killing my voice and it becomes lazy with no energy after a while. I am training to be a professional singer now, and I really want to get rid of this problem. I also have a lot of air in my voice.
Are you re-training your body as you re-train your voice?
You mentioned breathing from your abs, are you Supporting that breath? standing tall, feet shoulder width apart, shoulders rolled down and back, with the tummy tucked in pulling the navel to the spine... if you start to feel tense- move- move your neck, do some shoulder rolls and arm swings.
Get back in the swing of doing theatre style full body warm ups along with your vocal warm ups. Refocus your general exercise around core strengthening (all your power is in your core) think Pilates, with a bit of cardio for endurance, like Zumba (core focused cardio).
What you described sounds like the vocal version of trying to open a jar with your wrists. It starts to hurt real quick. When we stop, shake it out, breathe and use All the muscles in our Backs, that jar opens no problem. We only need to Tap into our Power.
As you are training to be a professional singer, you will be regularly meeting with a good teacher. Listen to what HE says, not to our random solutions when we haven't even heard you sing!