First, the proper stock fit is absolutely essential, both to improve scores, and to lower felt recoil. The biggest cause of felt recoil is improper stock fit, both from the stock rise and length of pull.
If you're getting serious about trap shooting, you can find a used Rem 870, or 1100 trap gun that will work, and both of those will shoot high, so you can see the bird going away from you. Your 870 Express is a field gun, and not designed to shoot as a trap gun. The comb is too low, and the bird can disappear below the front bead. If that happens, you can never break it.
You can also probably find a Browning BT99, new or used, which is a great trap gun for the beginner through mid-skilled trap shooter (I know a couple of excellent shooters who still use their BT99's). They come with fixed and adjustable combs, and a good stock fitter can make it feel proper for just a couple of hundred dollars.
I started out with a Rem 870 Wingmaster, found a Sile adjustable trap stock for it, and had the barrel drilled for adjustable chokes. I still have it as a back-up gun.
When I got back into trap shooting in 2007, I hadn't shot it in over 40 years. I got serious in a hurry (I had just retired). After I built up the 870, I decided I needed a break-action trap gun. I went to Jaqua's in Ohio, and invested in a used Perazzi MT6. I used it for about six months, broke my first 25 straight the day I bought it. I went through six more guns before I settled for an SKB 85TSS combo gun. I still have it, and it's had 150K rounds through it, and have only replaced two springs in that time.
If you don't belong to a club that "specializes" in trap, you should consider finding one. Those clubs usually have instructors, folks who can fit stocks, and can be a good source for used trap guns.
Also consider lessons. They'll help build your confidence, and lower the number of birds that hit the ground whole.
The "getting in" cost on trap, skeet, five stand, and sporting clays guns will be roughly $1.5K and up for a dedicated gun, including fitting.
If you ever get the chance to visit Jaqua's in Ohio, you'll go nuts looking at all their trap guns. They have the largest free-standing vault in Ohio, and it serves as their showroom. They have trap guns that run from $400, and going up to $50K, and above. Until a month ago, they had a trap club behind the store.